I don't think UU is that hard. I recently bought Emerald and am easily beating the game with underused Pokémon (Swampert is exception). However, you can easily be surprised by the power of Pokémon like Dewgong, and then may even win tournaments thanks to them!
All-ground is interesting, but a lot harder. Wallace, for example will be extremelly hard and you'll need to train your pokémon a lot more than usual. It will take more time...
Beating the game like you usually do isn't funny, but it's easier. Don't do it if you wanna try something different though.
Using only one Pokémon... I tried that, and it was so EASY!! I beat the game in less than 15 hours ( I think), and I take a lot of time usually. I did it with Torchic, and I recomend you also do it, but you can also try with Mudkip. But starters are better anyway. However, if you need a challenge, don't do this.
This is what I think. Even though using only one Pokémon is easy, I recommend it. You can have an excellent Pokémon in your game and since it doesn't take a lot of time, you can trade it to another version and restart.
Then use UUs. You can be easily surprised and such. Or you can choose to train only gournd-types, but as I already stated, it's very hard.
So, first with only one Pokémon, then UU. My opinion.