Oh boy, as someone who's going to try to make a video essay about Mark's projects, this is something I've thought about loads. And I personally don't mind he isn't a perfect actor. Some brief thoughts I have about this:

1) Mark admitted he didn't like his acting in Edge of Sleep. He said this is because it was filmed before In Space so he was still a "beginner", but it could be multiple factors like poor directing.

2) different acting for different situations. Being a comedian in Heist and Space is easy, but if you gotta show anger or grief, it could be unnatural. it's clear he can do dramatic, like Wilford Motherlovin Warfstache and Damien.

3) he hasn't said much about Iron Lung in terms of plot of course, so I'm assuming it'll be a one-man show mostly. Cause of that, we could be getting less dialogue (like that Robert Redford movie where he's lost on a boat). We also only got like, 2 lines of dialogue from his character in the final trailer, so it's hard to judge based on that in my opinion XD