Let's be real: the difficulty in Zero 1 and Zero 2 was honestly bullshit. Zero 1 especially for punishing you for using one of the game's main mechanics (cyber-elfs). 3 is where things actually got more balanced.
I dunno. Cyber Elves were a thing that I always considered to be a nice "option" for a hard action game of this sort. You wanted a sub tank or longer life bar to help you live longer? Fine, just take the rank score penalty to go with your power-up, and keep it rolling. (And in Zero 2, that just meant you had to make do without the EX Skills from the boss, but at least you still got the Elemental Chips to work with.) I could see where Inti was going with the idea of making it so the player would eventually not "need" the power-ups for future playthroughs, which is kind of a thing that MM gameplay design is all about, and I actually appreciated that. It also made for a variety of playthroughs. After all, there were specific unlocks tailored for those who went through the trouble of finding and using every Elf, too.
Rather, it's the need to level up weapons in Zero 1 and 2 that I considered truly bullshit. It basically meant that you either a) grinded against something like the towers in the first few stages in order to get your new weapons up to par (and say goodbye to your time ranking for the mission, as a result) or b) you better be willing to be hyper focused on your two main weapons so that at least they could be fully maxed out by the last few stages. Otherwise, you were stuck with a neutered move set, and who wants a Z-Saber without its triple combo?