First of all, the game is fun to play, and I hesitated a lot when it came to recommanding the game or not.
Be aware that the campain is like 4 to 8 hours long (it took me 5 hours and a half on hard difficulty) and the whole plot could be summarized in a line or two without skipping anything.
I wanted to play it before hopping on the sequel and I feel like I could have saved 9 bucks by watching a 5 minutes YouTube video, which is kinda disappointing.
DO NOT pay full price for it.

Edit:
So at some point something clicked and I started enjoying the game (still full solo, I never tried it online yet).
I guess I got gud, even got a few wins in a row and first tried Fulghor and Gnoster in their everdark versions!
(yay!)
I'm leaving my original (kinda salty) review (about 10 hours at that point, 40-ish now), and adding my updated thoughts about each matter.

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I cannot play online at the moment due to an unstable connection, so I can't properly judge the trio gameplay.
That's finally fixed but I'm so used to solo that I don't really feel like playing with randoms... Maybe later.

I rarely—if ever—leave negative reviews, but the solo experience in this game leaves me no choice.

Solo play feels like a 35-minute runback—it’s incredibly painful.
Still kinda does, but it's less painful now? Beating the game once probably helps since you don't feel like you "have to" beat X boss anymore...

I wouldn’t mind as much if the bosses were all well-designed, but that’s unfortunately not the case.
Well, it's not the case in any Souls game, Nightreign is no exception.

It kinda makes sense for a 3-player game to have two or three bosses at once, or a boss that summons minions—after all, you have multiple characters in your team to deal with them all.
Yes, this kind of boss existed in Elden Ring, but most players agree those were the least fun, and thankfully, they were quite rare.

In this game, however, there are far too many such encounters for it to be enjoyable solo.

They may have toned down the aggression system in solo mode so that some minions or bosses won’t attack for a while… until they suddenly do and the fight turns into a huge mess for a few seconds.
That might have been the biggest issue I had with the game at that time, especially since the first day boss of the first expedition is a double boss with minions, and the first night boss is, also, a multi-boss...
Actually once you get used to it, the aggression system in solo isn't terrible: in most cases it will end up in 1v1 with other enemies just enjoying the duel from afar. Just be carefull not to accidentally aggro them and you should be fine.
Getting gud helps.

The camera tracking is a lot worse that in the orignal ER, which makes those fights even more frustrating.
It does suck, but you get used to it. Mostly you need to act like the right joystick is actually a d-pad: only use the left/right or up/down directions, the rest (like trying to aim top right) is ignored by the game for some reason.

The randomness also leads to a lot of frustration, since bosses are mostly randomized. At the end of each day, you might face a boss you enjoy fighting—or one you absolutely dread.
I used to be way underleveled on each run, so boss fights could be really hard depending on the boss. Correct routing leads to better level and bosses, even the one you "absolutely dread" become no big deal.

The roguelike experience is almost nonexistent: losing a run just feels like you wasted 40 minutes of your life, as most of the “rewards” are meaningless... Who cares about a +1 arcane relic?
Relics do get slightly better, but I feel that it still isn't a game changer. Some can feel mandatory (I'm looking at you, evergaol relic), but most are incredibly useless.

That said, I have to admit—the Executor is very fun to play.
Ahem... I still haven't tried any other class besides my few first runs with Wylder...
It’s a shame I have to endure everything above just to enjoy that part.
A grain of salt.

I would love to update this review later and turn it positive after some very needed solo patches.
I guess I got patched before the game did.

TLDR: the solo experience really was an afterthought, tuning down the bosses HP and tuning up the experience gains is not enough.
Yeah solo was an afterthought, but it looks like tuning down the bosses HP and tuning up the experience gains might be enough.

Yeah I know—git gud.
I knew it all along. :)

If you liked Hades, you'll love Hades II.

I just made a race as Buzz Lightyear in Luke's speeder against Lightning McQueen and Martin.
10/10