Compare Halo 4's storytelling to Halo CE, 2, 3, ODST, and Reach. It's better than those four game's stories combined as they are "FPS drivel" as you put it.
Instead of the typical "super soldier saves the world" story that Bungie used in every game, we experienced something much more in-depth, complex, emotional, and believable. I'd argue Halo 4 actually had the first real story out of all the games, as they weren't really stories before other than MC having the ultimate character shield and being able to resolve any conflict. He was not like that in Halo 4. For the first time ever, 343 made Master Chief human, and it was truly incredible.
As I said, it's entirely possible that I could be judging the game poorly because I only played it once when it came out a while ago. And yes, H4 did make Master Chief as an actual human. I don't remember it being truly incredible but again, it's been a while.
Also writing is writing. It doesn't matter if you come from BioWare RPGs. It doesn't matter if you come from CoD FPSs. You either know how to create compelling plots with interesting characters that you make people care about, or you don't. The new head writer for MENext proved that he can make an emotional and engaging story, something I could never say about Halo before. He made Halo's story serious and credible for the first time, which was something I never thought could truly be done.
I have to disagree with you here. Different genres inherently call for different (albeit only slightly) writing styles. Yes, they both involve characters and conveying emotions, and I'll defer to your judgement of H4 and say that Schler can probably do that stuff just fine. The distinction emerges from pacing, choice, and scale.
An RPG is a much slower game than an FPS simply because of mechanical differences. The same story told in an FPS will likely be longer if told in an RPG, simply because of interactive dialog, emphasis on exploration, and investigation (not that an FPS can't have any of these, but H4 most certainly didn't to any great extent). Maintaining the player's interest in the story over longer periods of time is, quite obviously, harder to do.
Then of course, there's the problem with choice. Writing a decent story with various substantial choices is quantifiably more complex. ME3 writers even failed in this regard. So yes, the bar is pretty low, but there is still a bar.
And now scope. RPGs (especially Mass Effect) generally have a larger main cast and a much deeper lore (but knowing Bungie, the second one might not be true in this case). Incorporating lore is a tough enough task for anyone, and someone who hasn't worked with this kind of scope before may have a bit of trouble.
I am not saying Schler can't do any of this (he probably can). I am saying that he's up against something he's never fully faced before (at least according to IMDB). I'll say it again, he'll probably do fine, but I'd be much less worried (and I'm not even that worried) if I knew he was flanked by veteran writers who know the lore and have written "Mass Effect" before.
This guy joining BioWare is in the exact place he needs to be. Drew Karpyshyn isn't affiliated with BioWare anymore since he left when SWTOR released (he was the lead writer on KotOR and ME). Sometimes change is good, and certainly many would argue that there needs to be change after the way ME3 ended. The entire leadership structure of MENext is new and does not reflect the team that made ME1-3.
I'm not asking for the same exact team (or Drew Karpyshyn, mostly because I heard his books on Revan weren't that great). What I am asking for is that the team be composed of at least a few people that know Mass Effect, and that know what makes it tick. I'm sure that's exactly what will happen, but I don't yet have confirmation. All I'm saying is "Hey Chris Schlerf, you seem like a decent dude. I wasn't a huge fan of your previous work, and you have a task ahead of you that would be complex for any lead writer. I hope you don't let me down (you probably won't, but it is within the realm of possibility). Sorry I talk so much. Goodbye."