I think I figured out what I dislike about the game so much and why its mechanical design is overall so shitty.
Any good action game (you can most easily see this in character action like DMC and Platinum games) will treat the enemies like their own entities. They have their own attacks, habits, and tells which make them behave believably and as though they were actual people/goblins/robots/whatever. It's a bit tricky to put this into words without having a MatthewMatosis-level explanation of what I mean, but in a good game you end up thinking
and not
You can hit Jetstream Sam whenever you want in MGR and your attacks will be treated normally. The player always has the option to damage Sam, but some options are better than others.
Or, if this isn't truly the case, the scripting is so complex and thoroughly thought out as to make the enemies feel as though they have AI which exists mostly independent of the player. Half-Life has very simply programmed AI, but there are so many ways the entities in that game can react to one another it feels larger and more complex than it actually is.
Furi's bosses behave like they're from a PS2 game circa 2003. The main reason this is so noticeable and not an endearing quirk (in addition to the game's stiff movement mechanics) is because the bosses can't always be damaged. They have very clear divides between when you may and may not hit them, and fuck you if you try to be creative. It's less an interaction with a system of logic and more the bosses rapidly/randomly switching their "let the player hit me" switches from on to off and back again.
The angel chick with twin crossbows (surefire sign of cancerous character design, by the way) is the most egregious example of what I mean. She's only invincible when she glows orange, but the rest of the time she's so far away from you and moves around so quickly that your gunshots are going to miss 90% of the time - effectively invincible, and very clearly saying "You can't hit me right now, so wait until I go through an attack pattern and deliberately expose myself."
The boss can fly around through the air if you so much as look at her while in shmup mode, but as soon as she gets into an attack pattern, it's VITALLY IMPORTANT that she take a second and a half to SLOWLY draw herself back to a standing posture? Fuck off, game.
You remember when people would joke that Dr. Wily and robot masters should just build a really tall wall to stop MegaMan from defeating them? Vid related is what I'm talking about: the boss, Mephiles, will shoot rocks at the player and the player can pick them up and throw them back. This is the only way for the player to hurt Mephiles, So why the fuck does he ever shoot rocks at you?
It's possible to squeeze in three or four normal shots while she's darting around, but most of the time you have to wait for her to do something stupid and purposefully open herself up so you can actually deal damage. Shitty, decade-old game and boss design, and I thought the industry had moved past it.