I'm one of the few people who really felt 9 was a lot harder than any of the older games, but I'm also someone who considers 2 fairly challenging, provided you're not abusing the metal blade. I think how much and how recently you played and mastered the old games plays into your perception of 9. Could just be me though. I play 2,3,4 and even 5 quite often, but 9 kicked my butt for 3-4 days.
I've been picking my way through the whole classic series in recent weeks, which is what led me to finally hit up 9. The level design is just plain categorically
different than anything that precedes it, and in a very bad way that sucks the fun out of playing because there's seldom any margin for error.
Weapon selection is... above average, I guess? They've done better, they've done worse.
Concrete Shot: Interesting utilitarian uses, but if you don't get it early you're going to run into a bunch of nerve-wrackingly precise jumps throughout the game that seem to have been built under the assumption that you've got the thing. It's also terrible as an actual weapon.
Jewel Satellite: hahaha what were they thinking this is stupidly OP
Plug Ball: Actually pretty cool. Good damage, good energy use. Has obvious limits, but excels at what it does.
Tornado Blow: Never been a fan of screen clearers. They're boring, and basically exist to let you blow past minibosses without actually fighting them. The enhanced jumping is cool, but the energy consumption is far too high to make much use of it, especially when you start with Rush Coil anyway.
Black Hole Bomb: Another good one. Stuff like this is a lot more interesting than the screen clearers as far as high cost, high reward weapons go.
Magma Bazooka: Not bad as far as buster replacement weapons go, but the single shot limitation is weird and hampers its usefulness. Charged shot feels like it's only there because Atomic Fire did it first. Overall effectiveness varies wildly between levels depending on whether you run into lots of small enemies or fewer, heavier ones.
Laser Trident: At least Magma Bazooka isn't just Buster++. Effective, but where's the design effort, the creativity, the limitations?
Hornet Chaser: Rockman & Forte's Magic Card in homing form. Like its predecessor it's weak as hell and mostly exists to let the level designers lock away some powerups.
Not sure what I'd consider the series' best weapon lineup, actually.