If you’re talking about averages, then UE is way weaker. S6 has one of the lowest “spreads” of any build, for any class, in the game, because it does so much single target damage and doesn’t rely too much on fishing. That’s why, if you look at the leaderboards, (especially at era 2019-1) they’re filled to the brim with S6 clears up to about rank 5 or 6, and then the top few clears are Rapid Fire (because this build capitalizes much better on density).
UE is basically the opposite. It’s squishier and relies a ton on density, which is basically a double-whammy: not only do you have something like a 1/20th chance of drawing a good map (big open areas), you also have maybe a 1/20th chance of drawing a good mob type (good progression + won’t kill you), which means you might need to spend as many as 400 keys before you find a really “ideal” situation.
From the perspective of highest clears, UE and S6 are very close, maybe even equal. In 2019-1, the rank 8 NA clear is UE: GR 128 with 4799 Paragon, while S6 came in at rank 6: GR 129 with 7381 P. And if the UE guy gained 2600 more paragon, to match the S6 player, he definitely could have cleared 129, too.
Over on the Asia server, in the same era, top for UE was rank 4, GR 133 at 9482 P, while for S6 it was rank 5, GR 132 at 9438 P. So here, UE actually came out ahead.
I wrote this last year about class / set / build balance. I just re-read it and I still think it’s pretty relevant. You guys might want to give it a look.
If you don’t feel like reading that, the major takeaway is that it is virtually impossible to balance builds/sets on both average clear and top clear. So if you want to balance UE and S6 on average clear, then UE will clear higher at the top end. Or, if you want UE and S6 to be balanced by top clear (this is essentially the case now), then the average for S6 will be much higher than that for UE.
So: give some thought to whether you want stuff to be balanced by top clear or by average clear, because you can’t really do both.