Wouldn't both the console and PC versions have to be the same in terms of mechs, loadouts, items, internals...etc to be able to cross-platform play together ?
The plan is for feature parity, so that wouldn't be an issue.
Assuming the population is enough to properly segregate MMR, I don't think the mouse/kb advantage over controller would be an issue for cross platform play. PC players would just gravitate to higher MMR. Upper tiers would likely be dominated by PC, but then there are still players like Xployt who competed very well with a controller so it wouldn't be PC exclusive up there either.
Biggest hurdle I can foresee would be hackers on PC getting into games with console players who wouldn't otherwise be exposed to them.