Pretty cool game ! It is indeed challenging but I wouldn't call it frustrating, I think it is pretty fair on the player,
that being said, if you take the project further, you should probably zoom out the camera, being killed by an ennemy which just appeared in the screen leads to the player being forced to memorize the level and ennemy location rather than adapting to the situation, and to me, the second option is when this game shines the most