It's amazing how fast Piranha Plant fell out of favor. I guess it's not surprising, as while spike ball and poison cloud are excellent moves, they're pretty much the only thing Plant has going for it. Everything else feels like it was massively overbalanced to make up for those two moves. Even its other good moves have a large "but" attached to them.
The stretch attack has huge range, damage, and super armor, BUT it's slow, the aiming is impractical, and it stretches Plant's hurtbox, resulting in some physics-shattering counterplay.
Utilt is fast and juggles well, BUT it has negligible range in front or behind Plant, making it mainly an anti-air (which Plant has tons of already).
Bair KOes super early, BUT it's slow to come out, has a relatively small hitbox, and has massive endlag (not as much as an air dodge, but it's still very risky to use off stage).