The Necrons do NOT serve.
They serve the Primaris... and you know it!
On the contrary, I think it’s the other way around.
That doesn't surprise me. It's the whole PT vs. OT thing all over again.
The Nightbringer is really just a generic god of death and destruction (with a tiny miniature for what he’s supposed to be), pretty much 40K Nagash. Nothing fantastically exciting there.
The Nightbringer is epic, full stop, period!
Plus he sports the best name GW has created for a character (hence my forum name). I'd take the moniker Nightbringer over the forgetful name of Szarekh any day of the week. You are correct that he is the god of death (and consequently a true force to be reckoned with). However, that isn't the negative that you are presenting it to be. Under the C'tan, the Necrons were a unified and significant threat. Now splintered, the current Necrons are nothing more than D-list "villains" to get their shiny metal asses handed to them by the Primaris! (at least that amuses me... if I can't have it my way then I'll vengefully (and selfishly

) celebrate as the whole thing burns to the ground!).
Also, don't forget that he isn't the only C'tan. He is contrasted by the Deceiver, the Void Dragon and the Outsider. Each of those three are very different from him, and from each other. The Void Dragon is most intriguing.
(with a tiny miniature for what he’s supposed to be)
That is simply a result of when the miniature was produced. Models weren't that big back in the day. It is cool looking model though, even if it is undersized by today's standards. Besides, he is a star god, the physical dimensions of his body have no bearing on his potency on the battlefield.
Szarekh on the other hand is a flawed but believable character who does what he believes is right for his people, punishes himself when he realises his mistake, and sets out to make amends. I’d much rather have a properly-sized miniature for him to be honest.
The C'tan are flawed as well. They were absolute superpowers but they turned and consumed one another. Now, with only four left, the question becomes do they unite or finish each other off? Can the once nearly invincible C'tan be brought down low (all because they essentially destroyed their empire)? Who will assume command of the unified force of the Necrons?
A long, long time ago...
WAR BEARS!!!
( Yeah, a juicy old world update)
This is going to make a lot of people happy.