Game creates a checkpoint before and after the Metroid fights.
Kind of wish it didn't do this, actually. Takes away a lot of the tension.
For example, in Super Metroid the Ridley fight was one of the most intense boss fights I've ever played, and a big part of that is the tension from realizing how much progress you'll lose if he defeats you. It's a huge trek back down to him.
Here, even if you lose to the boss, you just spawn right outside the door. In a sense it's a QoL improvement, but...idk. Wish there was a way to turn it off.
I played Super, Fusion, and Zero the week before this came out and this game is giving me a much harder time than any of those did.
I agree, this is far and away the hardest 2D game in the series. Not super tough, but definitely harder than the others.
Metroid Prime 2 is still the hardest overall though.