I think MBJ is actually being choosy just by the lack of project he has, I imagine the roles are pouring for him but probably been turning them down.
And I know many hates Fox but if you ignore NTers opinions, they were still headed to a good place with First Class, DoFP and even the Wolverine making plenty of money by Fox's standards. So if I were offered a role in an F4 franchise (which is starting over) where Trank is coming off Chronicle (and for MBJ, he had experience filming with him already) and a sequel was already announced and promised an XMen team-up in the future and Deadpool R-rated could be making cameos, I wouldn't turn it down either. I could see those promises and seeing Apocalypse being made very promising.
Like most script probably looks good on first read (with a some exceptions of course) but it all ends with the execution and by the time you find out, it is just too late to back out. Like I am reading the third act and the conclusion felt real short and that reminds me of IM2 and the final fight with Whiplash and how short it was. Those things are more difficult to gauge on paper until it is filmed and edited then you likely won't know that part will be that bad.