Hi! I would say the difficulty was medium, reminded me of old arcade games (I guess that was the intention actually). Not too hard but neither too easy. The goal to master the game is clear, you just have to get there.
Reg. what to improve, of course variety of enemies can always be increased, right? or different enemy patterns, or an extra layer of enemies further away.
But I honestly find that kind of cheap feedback, so I can also suggest you to add an extra mechanic to give the player a second chance.
Controls are hard to master because changing the direction requires some time, it is not immediate. So if you are heading an enemy, it is hard to not die. That the key of the game I would say, so all good there. But, you could add a mechanic to be a bit more forgiving with mistakes.
For example, you could slightly slow down the time right before colliding with an enemy, so the player has a bit more of buffer time to react. The feeling of saving the day right at the last sec is awesome and could spice up the gameplay!
Or you could add a "parry" mechanic, so the player can press a key right before colliding an enemy, so instead of dying, they would bounce back from the enemy and has now a bit of time to react and change direction.
Just random ideas :) I hope it helps, in general very good game!