As you stated, the controls of Rush Bros. are NOT perfect...
That is a MAJOR problem for a platformer. If this was still 1990's and both NEW & OLD platformers like Mario series, Sonic Series, Donkey Kong Series, Super Meat Boy, Crash Bandicoot Series, Rayman series and Splunky NEVER existed, then Rush Bros would be a "serviceable" game.
But superior platformers DO exists and there is NO EXCUSE for a platformer NOT TO HAVE TIGHT CONTROLS. Furthermore, the level design as well as the game mechanic of Rush is generic and uninspired.
Playing the game is not FUN (main reason why I play games) due to lackluster controls and level design. Getting achievements and Playfire rewards are just a bonus for playing a game while I have fun. Frankly, playing Rush Bros. was a waste of my time because of my lack of enjoyment while playing a creatively bankrupted game.
A game can be difficult while enjoyable. Dark Souls is difficult yet still a fun game because it has very tight controls and interesting game mechanics that define it. Rush Bros. has neither.
Everyone has different taste in games, and the reasons I listed above are why I think Rush Bros. is NOT a serviceable game, but a BAD game in a genre with higher standards.
Eh, there's a learning curve to the controls. When I say they aren't perfect, I'm saying they aren't flawless, aren't total perfection, which is very, very rarely seen. Never have I said there were bad. An hour in I had full understanding of the controls and rarely if ever died simply because the "controls sucked", I died because I made a mistake (and the game is brutal).
The game relies on you knowing how to slide-jump. With the default control scheme, slide is B I think and jump is A. A and B are not easy buttons to string together on a constant, consistent basis. You simply need to map slide and jump to your bumpers (LB and RB) and you're set.
I spent around 3 hours playing the game and honestly, I got very good at the game. If anyone here knows how the game plays, how the play the game well, and how good or bad the controls are, it's me. All this talk of shoddy controls is heavily influenced by people simply being terrible at the game. I don't blame you, it's not easy, but blaming the controls is silly.
As for level design, to each his own on that one. No two levels that I played felt alike, and the balance between flat-out speed, careful trap dodging, and puzzling, was well done in my opinion. Not to mention there were some unique elements that I appreciated, like sound bouncers that blocked you if you timed things wrong, or propelled your up/forwards if you hit them right, spikes that launch upon seeing you and require careful dodging or speed to get past, etc.
It's a unique little game, and while it definitely has flaws, it can be fun. If you suck at it, probably not.