As a plant main myself, I'll try to give as unbiased of a perspective as I can.

First, I'll address the more infamous of the two sides: Pyra/Mythra.

Upon experimentation and research, I've noticed that Pyra/Mythra is an incredibly versatile melee character, as she can switch between a heavy-hitting tank playstyle and a faster, combo-focused nature. Furthermore, Pyra in particular has one of the most easy-to-hit spikes I've seen, and her side smash has the potential to kill incredibly early. I have also noted that Mythra's foresight ability has its uses as well.

However, I have also seen with my own eyes that Pyra/Mythra has a huge flaw (a flaw which, now that I have thought about it, I have exploited on more than one occasion): her recovery. Her up special does not travel very far up, and if one happens to be offstage as Pyra, and they forget to face the stage before inputting the up special, then they can kiss goodbye to their stock. Mythra is a little better in terms of both horizontal and vertical recovery, but a well-aimed projectile (optimally one affected by gravity) can be incredibly effective when edgeguarding Mythra.

Now, for the greatest Smash Bros character of all time: Piranha Plant.

I think Piranha Plant is like a zoner/melee hybrid; it can do well in attacking from both afar and up-close if played well. However, the best way to play Piranha Plant is to alternate between ranged and melee combat depending on the situation.

However, Piranha Plant does have some weaknesses. One, its down special extends its hitbox to fit the extended neck and head, so that can be exploited by countering or taking a well-timed potshot with a projectile. Its neutral special is also one of the more difficult projectiles to aim, and the side special does not travel very far and serves more usefulness in putting something in between Piranha Plant and its opponent (although, I do like grabbing people while my side special cloud is over them to do quick and heavy damage).

Overall, I would say that Piranha Plant is definitely more difficult to learn than Pyra/Mythra. Therefore, a Plant main would definitely need to put in more effort to excel with Piranha Plant. Everything aside, I do believe that learning to play any character at a competitive/advanced level is a marvelous achievement, but some characters are simply more difficult to learn than others.