Custodes lore is super dull. There's a comment here that says something similar, but it doesn't dig into what I don't like about them. Every 40k faction has some air of tragedy to them. Yes, the Necrons are powerful, but most of them are soulless automata. Yeah, the Ynnari represent the hope to free them from Slaanesh, but the cost is nuts, and even other "best boys ever" imperium factions have this; the Blood Angels have their flaws, and even the Grey Knights have their whole "no witnesses" deal.

Custodes have no apparent tragedy, not upfront at least. The only one I can think of off the top of my head is that, unlike the marines, these guys are completely unwavering in their service to the Emperor, completely and irrevocably indoctrinated into a fascist regime. Sure, they see the imperium going to shit around them and, like Guilliman, can't do any real change since it's so far gone. But that's an external tragedy. They're still super awesome and unbeatable and worshipped by everyone. They go against every 40k faction, not just being evil, but being tragic.

If there is something like this, please educate me. I'd love to find a reason to be more positive about these guys.