Maybe he is absolutely shite at the game and still finds it easy.As long as you perform adequately to the level of content you're trying to do, nobody will give a fuck if you use a controller, a turbo gaming dildo or m+k to play the game. You put yourself at a disvantage by using anything other than M+K, so if you underperform because of that, you get kicked and replaced, as simple as that. It's up to the group leader.
As for the "controllers are equally good" argument, they are not, argument is over. Some people actually had the audacity to argue that in games like Call of Duty, so basically console shooters, a pad is better than M+K but no, that's not right and never will be. Even with console aim assist (basically slow-aim, a noticeable sensitivity drop when aiming over a player model) you will never be as good as an equally skilled m+k player.
A very big one. If he's not a cutting edge raider or bleeding edge rated PvPer, his opinion on class difficulty is borderline worthless because if he can't perform on a level of at least 90%+ percentiles in mythic content, he can't even say he knows how to properly play his class and use it's entire toolkit in a raid/pvp setting, let alone judge if it's easy or not. How can a shitter judge the difficulty of a class if he's... a shitter?
Granted the difficulty of mastering a class has gone a bit down since Mists of Pandaria, but it's still not caveman easy. There's plenty, and I mean plenty of people that are parsing grey or green on Heroic, which to me is laughable considering how Heroic is basically a stripped down version of the real encounter, especially when they claim that WoW is braindead mode at the moment. If it is so easy, why are they parsing so bad? Can't blame trash percentiles on bad RNG.