I did not mean to suggest he would be worse than Chromie. I meant to suggest that his unique "lack of role" would be an issue in uncoordinated groups. See Abathur for example, as you yourself mentioned. A thoroughly excellent hero who is often picked or banned in tournaments but see little to no play in QM or HL due to his dependency of having good teammates. A lot of the more unusual heroes fall into this spot due to a lot of people being unable to comprehend anything other than playing their own character, and their own character only. These heroes need people on the team to work with them, if not around them entirely. And due to the steep learning curve, not a lot of people play them and thus they do not understand them sufficiently to play with one on the team.
I foresee Medivh falling into that same spot. Not because he is bad, but because people don't understand him. At least Abathur has had the luxury of being in the game since the moment the technical alpha launched, so people have had varying stages of 2 years to learn how to react to his presence. Medivh's release is going to be a clusterduck. He did not get that Very Hard tag for no reason, and unlike the last Very Hard release (Lost Vikings just over a year ago), he does not immediately invoke the feeling that he's earned that difficulty just by looking at him on the shop screen. Both TLV and Abathur stand out in that regard.
As for the rest of your Chromie comments, I'll refer to what I said earlier. I see her strictly as a niche hero that should only be picked in certain situations and on certain maps. Not all heroes have to be universally viable. A lot of people seem to forget that, and her winrate suffers for it.