They can obviously beat other guys that don't use steroids, but there is little chance beating a guy that because of supplements can train twice as much, and gets a huge technical advantage from this, is bigger, stronger and has better cardio. Even at amateur levels, the supplements would give a fighters "years" of more training and experience than a clean guy.
I did kickboxing in my late teens and early twenties. It was a mid size gym, so all kinds of people trained there, from normal guys wanting to lose weight, to the good kickboxers competing on national amateur level. There were not one single competitor (that did fights) that didn't do anabolic steroids (Anadrol and Dianabol) and lower level peds like Effedrin, and I can assure you that none of the guys that were clean had the slightest chance to even be competitive with these guys.
I have a hard time believing that guys that made it all the way to the UFC with only a few losses have been clean their whole career. At some point they should obviously have met a guy using steroids, and that means they probably would lose.
The UFC only picks up the best guys, and in smaller orgs/at amateur levels, there are hardly any testing, and the best guys will probably be the guy able to train most, that has a strength advantage, speed advantage, heals fast from injuries, and has a cardio advantage... And that exact guy that is a champ in smaller orgs, and has few losses is the guy the UFC picks up.