From what I've seen of it, the majority of Nightreign is Elden Ring assets sprinkled with a new dash of paint; various enemies and mini-bosses given a new move or 2 to mix things up slightly, some bosses from past games thrown in as a nostalgia hit (Centipede Demon, Nameless King e.t.c) with the "new" content being the final bosses at the end of each run... which, to put it another way, could be phrased as a 30-minute runback for when you inevitably wipe.
The biggest issue I have with Nightreign is the replay value. I imagine farting around in co-op alleviates it somewhat but having watched people streaming it for a couple of hours I stopped with the impression I'd already become well acquainted with the meat of the game.
It does remind me a lot of God of War: Ragnarok when Valhalla was added to it; an addition of a rogue-like mode and a slight change of pace paired with some additional lore dumps while keeping the core/fun elements of the "main" game intact. The only difference in this case was that Valhalla was essentially a free DLC while Nightreign is a £40 stand-alone title.. and in that regard it makes all the difference. Maybe when Nightreign gets a 75% discount I'll consider picking it up.
Nightrein is a rogue like, so repetitive gameplay is to be expected with slight difference. That being said, I was scared of the same thing but so far I'm having lots of fun. Beat the final boss on multi with random and I'm working on getting to level 15 solo everytime so I can solo all bosses, two down so far.
Some tips : don't fight too many low level ennemis, focus on beatable boss at your level.
Church are really useful at first, not so much when you get use to it.
When you solo, merchant sales one life, rare merchant the other ( he's shown by an icon on map), last one before final boss, so you can have four lifes woth the auto revive once on boss.
Try to use a relic with the boss weakness, so you can have a weapon with the boss weakness. Make a huge difference
On second day you should focus on big NM, Castle or the shifting earth event if you have a decent party. Solo I just do shifting earth or the castle in the middle.
Last trick might be patched, but once you beat the 2nd night boss, you don't have to move to the final boss, you can still move in the night. Grace site are active so you can heal yourself and get potion back and kill the monsters that are still up or gather church, etc.. You have a 2mn limit before the ticking kills you too fast too heal. And you can get you rune easily back just make sure you dont die in a tricky place. It's easier to get used to move in the night when the flask give healing over time