The mechanics are sound, just frustrating. Jumping is controllable, and manage your double jump to redirect is something that wasn’t unheard of in the NES days. Enemies have consistent attack patterns and always spawn in the same way. Attacks don’t come from offscreen, so there aren’t cheap deaths, just deaths that you don’t have the time to react to due to being in an animation.
All of these things are present in games from the NES days, and from today, hardcore or not. The only things that make Volgarr difficult are the checkpoints which actually exist unlike a number of NES games, and the limited control in the platforming which actually makes some sense.