[Garganator]

Just stunned that there could be a poke that is actually better in Ubers than it is in OU.

Heck, there are pokemon, which are better in Ubers then in UU/NU. Perfect example is Jumpluff - it's solid in UU and good in NU, however in Ubers with permanent sun it's one of the most annoying pokemon to face. Perfect Leech Seeders and used on many quick stall teams with Groudon support. There was a time when Parasect was practically "only Ubers mon" however Kyogre users started to use non-choice sets, which made it much worse. Another prime example is Shiftry - with Wobbufett support really good sweeper, either when using Nasty Plot or Swords Dance. So much stuff is weak against his Grass/Dark STAB it's not funny. And quite gimmicky example, but IMO Ubers is best tier to use it with Sandstorm being quite rare (and hail even more) making it viable if played right. This example is Shedinja and I used it with some interesting results on stall teams, as Shedinja walls surprisingly alot of stuff in Ubers.

I imagine that Blissey can't thrive that much in Ubers since there are so many beastly mixed attackers running around.

Blissey in Ubers is as good as in OU, however unlike in OU you need to support your Blissey a bit more, as you mentioned many pokemon here goes mixed, however Ubers is much more special based, giving Blissey perfect niche in this tier. In other words Blissey user must predict with Blissey better then in OU and switch-out with her more often (for example you switch on Palkias Surf and good move is to switch out to steel pokemon, as in this moment Palkia may Outrage you to death. Anyway Blissey can't stay in as easily as in OU). Ubers is almost all of time "pure checking, not countering" as hard counters are hard to find in tier, where almost everything hits like a truck, making predictions harder.