to be positive, the pistons need a shot blocking big, preferably one who can roll to the basket and even shoot a mid range jumper. that's ayton. he's young. he should improve and be a fit in that respect next to cade. we saw bagley improve offensively when next to cade, but bagley plays zero defense and ayton plays some.
to be negative, big fives are growing obsolete, especially in the playoffs when teams go five out. only centers who can move laterally (like williams for boston or looney for golden state) or guys who are deadeye three ball shooters like horford or unicorns like embiid can stay on the floor in today's small ball shooting age. ayton was a non factor when dallas went small. also, there are lots of rumors about him being lazy and more concerned with his ability to score in nba2k than in the actual nba.
for the pistons, he would be an upgrade on stew, but at a big cost in dollars and potentially assets, as he is a restricted free agent and phoenix could match the piatons offer or extort players and/or picks from detroit.