Miz is a classic example of the swinging pendulum of opinion. Miz was terrible to start, and as a result was a bit underrated when he improved (both character work and in-ring). But of course, that got spun into "actually, Miz is great!" when he was merely better than conventional wisdom. The truth is in the middle - he's a serviceable worker with a character that has value, particularly in the modern-day WWE.
I'm not sure MJF is a ton better than him as a worker on the whole, but MJF certainly has a higher upside and has continued to improve. MJF has a ceiling unless he can demonstrate consistently that he can deliver a main event match, but he also has time to get there (I think he may have a similar trajectory to Jay White in that regard, where it takes him some time to really pull the heel work into the big match without it hindering the overall quality).