In general, we don't do balancing for RGM game modes, as their goal isn't to be a 'hyper-competitive battlefield'. We do draw the line in a few extreme cases though, as our goal is to disable no champs ever in RGM modes.
-- When champs can permastun an enemy, we make changes.
And I don't mean that as hyperbole, I mean, the other play actually doesn't get to click any buttons and play the game at all. EG: during dev, Zilean on URF, Brand on OFA, etc. This is simply unacceptable, but rather than disable the champion, we make a slight tweak and you can still enjoy the mode if you love Ryze, or whoever.
-- When the nature of a mode renders champs effectively unplayable, we make changes.
When a champ is so underpowered that they're just not fun to play, it's almost worse than having them disabled. You feel tricked after picking them, finally get into a game, only to discover your favourite champ is the anti-URF. :/ In the case of Bard, we already increased his chime rates last year, but we'll probably bump them along a bit more this year.
**Note on RGM champ viability: **
Not all champions are expected to be equally viable in RGM modes. Champions are designed, crafted, and balanced around SR. That is a place where the endless pursuit of balance should take place. In RGM modes there will usually be a few over/under performers, but we keep a close eye on them and you have 6 bans just in case. ^_-