This doesn't really explain why people think it is bad. Is it because someone in light armour can have the same AC? I've used medium armour on my dwarf ranger, seemed to work fine. I think people are making an issue out of pretty much nothing.
It means Medium Armor Proficiency - the feat and/or the class feature - is worth nothing without some meta-gaming with ability scores.
And, in a game-ist sense, it makes medium armor proficiency penalize you for having a high ability score in the most useful ability in the game, without any bonus whatsoever and higher weight of armor, when a higher feat on the 'feat tree' shouldn't do that.
IMO It should be better by some margin. That's what this is trying to fix.