Siren and Diabolos were fine IMO, because they had a bit of difficulty when released and were engaging fights. (I screamed like a fangirl when Diabolos was revealed lol.) And no one was really complaining about them being dungeon bosses either.

Fenrir, however, pissed me off, as well as lots of other people, with how easy, boring, and disengaging it was. My problem with Fenrir has little to do with him not being an official Primal; it was mainly because it was lazy especially when compared to Diabolos and Siren. Hell, back in the day people dreaded being qued into Pharos Sirius and would rather eat the 30m penalty then to fight Siren with randoms.

Diabolos on the other hand, was done perfectly. The fight is really thematic and fits Diabolos well, and has the perfect amount difficulty where it is beatable by randoms but you have to pay attention for the entire fight. That's exactly how these dungeon-based 'summons' should be, not snoozefests like Fenrir.

With all this said though, I hope they don't turn Shinryu into a dungeon boss. He has the potential to be the final boss of any future 24 man raid.