Depends, we didn't get bronze doing UBRS the first day but we did the second and were 2 seconds from silver. Most of the other ones we have gotten bronze on our first runs though, they aren't difficult they just have some small trick or something that makes the fight exponentially smoother.
IE First boss of GD we make sure no one casts during slam = ez mode. Second boss we put a melee on cannon, it negates slag and suppressive fire for the most part. Slag 100% though.
Auch from yesterday someone rooted the Fel on the last boss also which makes it easy mode. We wiped on the demon dude a few times with the little demon shitlings dazing people until we started to kill them.
Ao if you ever want to run let me know you are on my Bnet, im always running extra CMs with friends who need a tank or me to dps.