Beat can fire disorientating sound waves. That alone pretty much makes Gen godmode if he would use it more than ONCE.
Smokescreens are always useful.
Flashbangs even more so.
Water he's proven can be useful, since though the nozzle points downward, it still fires at a high enough pressure.
Pen materializes what he writes into solid metal.
Hand gives him a 3rd arm on his leg.
And Net can create energy fields to trap opponents.

Gen's switches are all strong in and of themselves, without requiring some convoluted combination.

See! -points-

Having a 3rd arm, writing in metal, and firing water still aren't incredibly useful in general combat without a context. I totally maintain that none of his switches are totally useless, it's just that he has to sort of be "creative" when it comes to how he uses certain ones. I use the word creative lightly because the trump card was always whatever switch had just been finished by Kengo, but it felt a lot like an inexperienced, lucky superhero should. I know you mentioned Spiderman as a guy who has to make do with a really simple set of powers, and while that's true, if you look at a series like The Spectacular Spiderman, he would always win out of some lucky bout of creativity, rather than sheer force. That's how a lot of Fourze's fights felt to me, even if his first instinct was to go in with Rocket and Winch.

:lol same here

Aero isn't that useless, I do agree with your point of Stealth being underused though.

The difference is Spidey makes due with his webs. Using it as a literal all purpose tool. Despite making use of it every fight, his fights aren't always just "Web opponent, use webs to throw things"
Fourze, on the other hand, always uses the same general tactic. While his Switch choice is different, Gen never deviates from the "Run at him" strategy. Rocket, Claw, Chain Array, Elec, Giant Foot, Pen. It didn't matter. The only time he changed up is when he used one of his 4 distance weapons (And even then, he was still running at them with Fire).

And I'll concead he powerspammed Cosmic. Never even tried anything else most times, he just went right for the switch.

Cosmic was a tease and, ultimately, a disappointment.
When he used it well, it was awesome. But for the majority of his fights, Cosmic would only show up for the Finisher (i.e. THE LAMEST PART).

Well, keep in mind that the Fourze suit wasn't designed for combat, it was designed for space exploration. Ostensibly all of the switches should be useful in some way for an astronaut - key word "ostensibly."

I heard a theory - probably on HJU Radio - that Kengo's father designed the switches while he was dying on the moon and that some of the stranger switches were the results of him running out of oxygen. I like that theory almost as much as the one that Yuuki's increasing dementia is a result of standing too close to Gentaro when he transforms and getting exposed to cosmic radiation.

You're generalizing it. Spidey's fights can be summed in "Try hitting enemy, enemy is too tough, swing around and hit enemy with object in environment, web up opponent". If Spidey had a weapon for every random crane, cement mixer, metal pole, what have you, it could be turned into the same complaint you have with Fourze. The difference is that Fourze has to sell 30+ melee weapons, while other heroes that have to be creative with their all-purpose tools are free to use whatever else is written in.
Fourze did some cool stuff with his weapons too. Using Board with Winch and hitching a ride on the Meteostar, using Shield, Aero and Scoop to try and capture the Fly Zodiart, everything he did in Cosmic forms debut. If you could consider the Fourze Driver his "all-purpose tool", I don't see how that's very much different from other superheroes.

I already know I'm generalizing it. Ichigou had a stick. Den-O had a broom. Fourze would have a Stick Switch and a Broom Switch.

I have a bias against Fourze because he fights in the exact manner that I hate. "Did hitting him really hard work? No? Hit him again with a different switch." Repeat ad infinitum.
That, and coupled with his outright refusal to use any instantly non-offensive switch (Turn invisible with Stealth? Fire disorientating sound waves with Beat? Blind opponents with Flash? Nah. Just use Rocket) just annoyed me.