I agree that Giratina is probably still too good for OU. It has a few notable counters, like Lati@s, and Griseous Orb means it is quite prone to be worn down. But its stats are very impressive, and no type that hits Giratina super-effectively resists its attacks.
Dialga I'm 99% positive that it's not Uber material. It's worse than standard Kyurem-B in every way. It has worse stats and much much worse typing.
Palkia is 4x weak to Water and Fire attacks, but I honestly don't see many Water and Fire attacks in Inverse. The only Pokémon that commonly carry those moves are Dragon-types for coverage (which are going to take Palkia's Water attacks much worse). Water-types are weak to Water themselves, so they are no counters, and Fire-types are generally not very good.
adman2 raises a good point about Lugia. It gains a 4x weakness, which could be problematic for a wall. However, Fighting-types are very easy to wall and are weak to Stealth Rock (while Lugia is now resistant to it). I'd say they are going to be worn down much faster than Lugia is.
Darkrai may be weak to Psychic and Ghost, but they aren't really more common than Fighting is in standard. Also, Psychic- and Ghost-types are both weak to Fighting, while Fighting-types aren't weak to anything that standard Darkrai carries.
Standard Darkrai also has a move syndrome that Inverse Darkrai doesn't have. Fighting + Dark is literally unresisted in Inverse.
Bottom line, I think Darkrai only improves in IB.
Mega Blaziken, just like normal Blaziken, has one glaring problem: Ice-types. Ice-types resist both Fire and Fighting and hit back with SE attacks. The fact that Blaziken is now weak to Ice Shard and Sucker Punch doesn't do it any favors either.
Another smaller issue is that resistance to Fighting is quite common and it forces Blaziken to use Flare Blitz, wearing it down. The one time I happened to face a Blaziken without Ice-types in my team, I sacrificed one of my Normal-types to take Blaziken down with its own recoil.
EDIT: In the true spirit of Inverse Battles, I actually lose MORE when I use Ubers. This is partly due to me being overconfident.
I don't know what team you've been using, but I believe part of the problem could also be that Ubers tend to share similar weaknesses. A team with a couple of Ubers and then the rest of the slots dedicated to cover the Uber's shortcomings is probably going to perform better than a 6 Ubers team.