I have now tested the companions fairly extensively in numerous types of cube both with and without rule changes. They were pretty foolish without the extra 3 cost although it didn't make all of them playable as it could be quite hard to draft around some of the narrower conditions. Mostly it just made the good ones oppressive so I am glad they changed that. I would advise playing them as the rules now suggest with that extra 3 mana cost, if at all. While not as off the charts in cube as you might expect with all the fuss they received if played as written, the problem is that games become pretty samey pretty fast with so many of the same cards and interactions popping up time and time again. The common theme with the companions is that all bar a couple of cases are only worth running in a cube if you can have some expectation of having them as a companion. Without the potential to be a free extra card they just don't compete well at all with the alternatives. This article is an adaptation of a post I made (testing the waters!) on Reddit about nine months ago. Since then Yorion has jumped up a position but otherwise my opinions are unchanged. As played in an unpowered cube with updated rules I would rate the companions from best to worst thusly;
Lutri 1st
Having no cost to fulfil the conditions in singleton makes him an auto include in any deck that can cast him. Simple as that. Best in burn heavy red where he is a free chunk of extra push, or control where you have the time to setup a good two for nothing. Bordering on the too good even if he doesn't even get put into hand in over 50% of games. I am very happy first picking this guy. I have now banned this from my cube for being too good for too little cost. It just ruined otherwise good games where resources are running dry or there is a bit of time spare. The ability to just get a bunch of extra free stuff is just too much.
Lurrus 2nd
Almost never a companion as you basically can't run planeswalkers or other high powered threats if you want to play Lurrus and that is too much of a price in cube. You just can't draft enough muscle to win with if you want to fulfil the condition. I have yet to have a good deck that could use Lurrus as a companion, they have all just lacked power and run out of gas. Even with Lurrus surviving and getting back some random two drop every turn isn't able to cope with a lot of more expensive cards just being in play doing their thing and/or attacking. Still amazing as a card in the 40 allowing you ongoing value and the only companion that is just a stand alone strong cube card that you would still play without the companion ability. Best in decks with Baubles and/or things like Selfless Spirit and Savoir. Commonly a white weenie or aristocrats inclusion. Ultimately quite a midrange sort of card not a million miles away from Meren of Clan Nel Toth.
Yorion 3rd
In a draft it is very hard to get a viable 60 card deck to then make this your companion, although not impossible. If you go to more than 45 picks it quickly gets a lot easier. In sealed it is more commonplace to be able to make 60 card decks for Yorion and as such it is one of the top tier archetypes. You don't even need a flicker theme in a 60 card cube deck for Yorion to be great, just a big flier for the late game with no card cost is plenty good enough. Yorion is passable in the 40 as well although not the most exciting. The card has steadily improved in cube since release as there have been more good flicker targets on top of the format tending towards slower and grindier games. Well worth including if you sealed a lot, draft pools over 45 cards, or have a flicker theme. Otherwise not.
Jegantha 4th
A surprise performer. This is quite easy to fulfil the condition for and gets used as a companion a lot. One of the only companions you can easily draft around supporting. Good gold cards tend to cost such that they comply with Jegantha. As such it is commonly found in red based Fires of Invention/Breya artifact leaning four colour gold decks and the more conventional green based four (Omnath) and five colour good stuff decks. Providing massive mana outlay and being a meaty 5/5 give Jegantha two distinct and useful modes, that of fatty and that of huge mana output. While a common companion Jegantha is rarely (never?) used in the 40 simply because it is well below par. You just don't want a vanilla 5/5 with no value that doesn't do anything else unless you untap with it. We certainly are not playing five drops just for ramp! I have cut Jegantha as it doesn't add that much to the cube experience. It wasn't played often enough, playing it rules out other cards, and it is never making a main deck so kind of feels like a utility land pick. Something that doesn't add to your count of playables and probably won't actually do anything significant. Power wise the card is there but in terms of what it actually brings to the table I am rather less interested.
Obosh 5th
Doable as far as fulfilling the companion conditions and still pretty playable in aggro decks when you don't. Playable but not great. A kind of bad Torbran. Not a bomb by any means nor that commonly seen. To draft as a companion you need to see early in the draft. To just pick him up late and run in your aggro deck likely means your draft didn't go that well! Not a card I would bother with purely on power level and action but it is quite fun and novel. Paying three extra mana on your five drop isn't exactly the rip roaring tempo a cube aggro deck can get that exciting about.
Gyruda 6th
Not really powerful enough if played in the 40 and too much work to get it as a companion. It is not that it is any harder than Obosh to make as your companion, just that you also have to build a deck that wants it and that is good for Gyruda as well. That isn't something you can do easily regardless, let alone when you rule out half your potential playable cards. You either need a load of really good four and six drop dorks in your deck or you need top of library manipulation so that you can setup a hit. Without this support the odds on Gyruda hitting something meaningful to recur is going to be low. Supportable if you like and best in midrange reanimate decks. Not worth the effort in general however.
Zirda 7th
You can use this as a free card in control decks with few permanents and thus a fairly easy condition complete. You can also run this if you want to run the infinite mana combos with it (with Grim and Basalt Monoliths). You cannot run it in red and/or white aggro decks as they want to run far too many permanents that rule out Zirda. It is also only worth running if you can have it be your companion with the possible exception of the infinite mana combos. None of these things are that good, common, or importantly that exciting. The upside of a random 3/3 dork in a control deck that costs a total of six mana really isn't worth the hassle of supporting. Just not playing a Wall of Omens might well reduce your EV more than Zirda can add.
Kaheera 8th
Not enough support to run as a companion in limited cube and not good enough to run normally. This is all down to the number of cube playable dorks in the appropriate tribes. You can sneak Kaheera into some creatureless control decks a bit like you can cheat on Zirda. The problem here is that creatureless control decks, unlike in contructed, are really hard to pull off in cube. You limit your options and give yourself a really hard time of winning. The upside of a zero card cost 3/2 is hardly impressive payoff. I have built a tribal cat list with Kaheera as a companion and there it was outstanding. Having a lord as a companion is quite a boost to a tribe. Cats is also a rare tribe in singleton in that it has loads of viable one and two drops but fairly little payoff for being in the tribe. Kaheera capitalizes on this perfectly.
Umori 9th
Easy to fulfil the conditions but it is a trap. It is really hard to be competitive in your inevitably midrange dork deck without any non-creature spells. Your disruption will be sparse, low powered, slow, and sorcery speed for the most part. By the time you cast Umori the cost reduction is near irrelevant and so you just have a very costly Ernham Djinn. Umori is too lacking in power to entice as a non-companion inclusion and so with the companion mode being a trap there is little room for Umori in cube. I imagine there are some singleton constructed decks that are naturally very close to all creatures that could wind up running Umori for free but it is not like he is going to add all that much to those lists. You might even just prefer your sideboard slot...
Keruga 10th
Laughably bad if used as a companion as your deck will just be clunky slow horror. If you are never using as a companion then Keruga doesn't have the power level to include. You can just about build a constructed deck with suspend cards and split cards and the like that let you have things to do with one and two mana so as to allow Keruga as a companion but it is then just a bit more of the same midrange value stuff. You might as well just play better cards and make up for the loss of free Keruga value that way. Keruga takes the wooden spoon by being bad both supported as a companion and if just played normally. Umori at least has some corner cases he has potential in. Umori also comically won a constructed event I did involving a deck built around each companion and using the original rules. To balance the event we bid on the decks using starting life and cards and Umori just won thanks to having a bunch of life and cards over opponents. Umori wasn't quite as bad as everyone thought!