We need to consider thar Mangini came to the band inmediately after the only drummer for the band at the time, a founding member who had been in the band for 25 years and was integral to the production of the albums, quit, so MM didn't only have to take care of the drumming (which is already a quite difficult task), but also had to endure all the pressure to please fans all over the world who have a very high standard of what to expect from the Dream Theater drummer. Not only that, but he still has to deal with all the comparisons, bad comments, etc etc that people say on almost every post by the band, plus, all the bad comments about him and current DT on Portnoy's social media accounts, that MP usually gives likes and RT. That's not a light weight to carry.
Having said that, I think MM has done a GREAT job dealing with all of this. Just imagine if any other drummer got the gig, I'm not entirely sure he could've taken as much as Mangini has. This just shows how much dedication and hard work he has put into DT, and, regardless if you like him and his drumming or not, we all should respect that.