I've played the normal campaign on nightmare around 15 times and I've ended 2 UN attempts on final sin. TAG1 however, is less fun to replay. I love the blood swamps and parts of atlantica, but the holt is forgettable. The addition of the blood maykr was kind of dissapointing. I just felt nothing when I fought one of them for the first time. The fact that the blood maykr is a traffic light enemy makes it kind of annoying and less usable in a variety of fights. Same goes for the spirits, I felt nothing except for some slight irritation that I had to pause my dance and focus on this one blue sucker with my ghostbuster-beam.
TAG2 is probably something I will enjoy, but I don't think it's going to blow my mind like Eternal did. I would rather see a new game where you start over from scratch and gain utilities as you go. Like they did with doom2. Add a couple of new monsters, some new weapon and make new interesting maps. Also, less focus on story so people can stop arguing over the quality of lore blah blah blah. I love a good story, but when it comes to games like this (or games overall for that matter) I just couldn't care less. But I guess they are stuck with expanding on this story now, since they've put so much focus on it.
I find it quite fun that they are focusing on this super hard gameplay. I have always been quite masochistic towards myself when it comes to games, so it's a welcome thing for me. You can call it niche and gimmicky, but you can always lower the difficulty. Hell, you are almost immortal on ITYTD.
About the unmaykr: In horde mode, you only get the unmaykr and no BFG. There it works wonders against archies and barons! I'm having a hard time coming up with how they should have made the unmaykr a better reward in the base game though.