You can have both fun gameplay and consistency. In terms of gameplay, Super Mutants are just big, tough humanoids, nothing unique. I do like what the addition of the suicider in 4 does for gameplay, but they're bullet sponges in most of the games. Bethesda proved they can come up with a regional variant on that enemy type in Point Lookout with the Swampfolk.

As for the Super Mutants themselves, they have at least half-assed an excuse every time they appear in the games, with 76's Super Mutants kinda fitting pretty well. The more they do it, the less valuable FEV is, but the Super Mutants are really the least of my problems with the lore. However, their constant reuse indicates a desire to establish series icons that they can use to market the game instead of good mechanics (look guys BoS again!). Above all, it really indicates a lack of creativity to me. They absolutely did not have to make a choice between their game being fun and their game having well-thought-out lore.