I'm a member of the Smash scene. Are you calling me a pedophile? If you're not - and you'd better not be - then stop generalizing and read the fucking thread.
UCF works on unmodded hardware, yes. The vast majority of consoles at Melee events (and probably all of them at most events) are unmodified Gamecubes, and as you note UCF is loaded using a memory card exploit.
The problem with Nintendo's perspective re: UCF is actually laid out very clearly in Reggie's answer to that first question. He says Nintendo wants a level playing field, and that means no UCF. Well, that's just completely wrong: the only thing UCF does is level the playing field by making a particularly volatile characteristic of a controller somewhat less relevant. He was also wrong that the new supply of Gamecube controllers would solve this problem since this new supply is equally susceptible to the problem UCF aims to address and even if it wasn't the controllers are generally considered to be of lower quality anyway.
If Nintendo had their way and everyone dropped UCF, that would be a problem to the level playing field for sure. A good controller is already easily over $100 and the supply ain't getting any bigger.
It's at least in the hundreds of thousands of people, and that's saying something considering the game is nearly 20 years old, is quite difficult to buy and get into now, and receives less than zero support from its developer.