The issue with the game is the issue with most JJP games, rules design. DI suffers from this as well. GnR is on the lower end for me with regard to rules design, but it's at least a 'decent' rating. I'm not a fan of too many multiballs, especially when they feel like they aren't earned. The DMD era was the best for multiball balance imho.
As for Ultraman, I have that game. It's the first game I only played for a couple days and pretty much stopped. Is it as bad as Thunderturds? WTF? Are you kidding? I've played Thunderturds. UM is actually pretty fun, but the code is still at a level that it shouldn't be shipped. It's a game in early development and should have had at least double what's in it before it was shipped. It'll probably be okay/good when it's done in a year or more.
All that said, believe it or not, Ultraman with it's bare-bones code is more fun to me than GnR. It doesn't have the crippling rules design, and it has really fun modes and surprisingly fun mini-playfields. I can see that it 'may' turn out to be pretty awesome or it 'may' turn out to be pretty average.