I've started playing the game again, and I can definitely say it is HARD. I'm in Kraid's lair and I'm not making progress. I believe I have 2 energy tanks and around 20 missiles, along with morphing ball and bombs. I've also gotten the long range weapon, along with ice beam.
I've mostly started playing it because we have Metroid Fusion now, and I'm going through the metroid games in order for some reason.