At least in FMA they actually worked with the author to make a story
Fillers are generally fine as long as it doesn't interrupt the story and are not too long. Bleach has that issue a lot cuz their story arcs are pretty long and it tends to get interrupted for filler. Naruto had the longevity issue cuz their filler just lasted way too long. If you can provide meaningful filler too, that's a bonus, but I generally don't expect much.