I am currently trying all companions except Lutri and Lurrus as occasionals. The problem hasn't been that they were too huge power-outliers – But so far they have only been maindecked. I am talking about the good ones like Yorion, Umori or Gyruda. I think for lower power levels, especially when we're talking about a limited draft context, I would have preferred companions to be weaker at face value but still come to your hand with the old companion rule.
Imagine if Yorion was only a 4/4 and would only blink one permanent, but he could come to your hand for free if you build around him. I think that would have been more appropriate for my cube, at least.