While RDJ was on the rise, in 2008 Iron Man certainly catapulted him to new heights with the moviegoing public - in terms of having a wide mainstream appeal (he was always a very good movie actor, full stop).
Hemsworth - yeah, I feel that without Thor he'd be struggling to have achieved leading man status - and Hiddleston's work is more acclaimed (I'm thinking of the Night Manager, more than Kong). He's great as Thor, but just kind of average as anything else. Hey, he's a soap opera actor really, so we can't expect too much.
Evans....should probably get more credit for making Cap, who's kind of a dull character quite likeable and someone we can identify with (which ain't easy for a guy whose superpower is being human perfection itself). Has it hurt his career....nah, I mean it will probably be the role he's remembered for, but is he as versatile as RDJ, I don't think so. Finally saw Snowpiercer, and it's great, but a very strong supporting cast help carry it along.
Pratt, I feel like GOTG really took him to the next level, it was the perfect role in the perfect film. He's probably benefitted from the exposure more than any other actor.
Scar Jo, well she's had acclaim for a long time. I think BW keeps her current with a particular type of audience, but I think she'd be working lots regardless.
Paul Rudd is getting a serious career boost from Ant-Man, as is Anthony Mackie.
Sebastian Stan...well if it weren't for TWS I would have no idea he even existed.
Renner.....hmmmmm. A guy who can really act playing sort of a sidekick role. I don't think the MCU is hurting him too much, it keeps him current and relevant.
Has anyone mentioned Cumberbatch yet ? I mean, a guy who's got a ton of acclaim for Sherlock now looking safe as a Hollywood leading man after Dr Strange.
Not sure what will happen to Boseman, will just have to wait and see - but no doubt he got quite a buzz for his appearances in CW.
Mildly embarrassed to admit it, but I don't even know who Brie Larson is, so I've got to say that being a major Marvel superhero will help her career - as I'm probably not alone. Then again, when I hear "Captain marvel" I always think of Shazam.
In summary, I don't think being in MCU films hurts anyone's career too much - quite the opposite (particularly if that person was already well established) it gives them exposure to a massive PG-13 audience, who may grow up to enjoy their other work.