We've seen Saiyans eat insane amounts of food, but where does all that food go? It can't fit all in their stomach. And if we assume they digest it at a crazy speed, shouldn't they poop while eating?
-Recoome is durable and can win if he fights smart, but he's too stupid and loses.
-With Piccolo, it can go either way.
-Yamcha loses, because he's Yamcha.
-Roshi loses.
If they needed to train to increase the multiplier, obviously the fusion wasn't perfect yet. It's not unreasonable that Goku talked about perfect fusion (SSJ Gotenks post) when he said they could beat Fat Buu.