It would honestly be a bit too simplistic for me if you can walk up to your opponent, hit them, retreat, wait for them to hit you, and then spam attacks again. Then it would be easy to defeat an opponent. So you have to fake an attack by jumping over an opponent and then attacking from behind. Of course you can see that differently, and if everyone shared your opinion, that would also make me change this part of the game as soon as possible, but I would find it unfair to give me a bad rating because you find the game too difficult.
As for PlayerCollission, I use a normal Capsule Collider, which is very common for 2D platformers these days. Slipping off the edge of a platform is just realistic. I would find box colliders inappropriate here.
As for the double jump, you just have to time the double jump a bit. It's nothing that other games wouldn't do, that the timing of when you double jump varies the jump height.
The objection regarding the spikes is fair. But that doesn't make the game bad.