Because his friends forgot about him for x amount of years and appeared only in the end. As far as I understood, that's the main problem. Sure, you can headcanon that they didn't forget about him, but that's the point. Author didn't show it, and for a shounen like MHA it is important.
Doesnt sucks per say, but more like "geh.....what a shitty author, he ALSO just like other famous authors did a "safe ending" and not what the fans want"
Death note = oh wow Kira lose
Bleach = oh wow deus ex machina win
Naruto = oh wow naruto is hokage and he forgive everyone
Eyeshield21 = oh wow sena doesnt date anyone and somehow continue the sports
AOT = oh wow MC lose
DBZ = oh wow goku is sent away by dragon
Toriko = oh wow go to space
Shokugeki = oh wow SKIPPED RESULT OF MOST OP COOKING COMPETITION (if MC won or not)
etc
etc
Never a 100% satisfaction ending.