If you spend even a little time on the 40k sub, there are eternal battles that rage about certain plot points:

Did Magnus really do nothing wrong?

Why are the Space Wolves like they are?

Fuck Erebus (this one isn't controversial, but everyone hates Erebus, there's even a sub on reddit for it)

Why did the Emperor not rescue Angron's people (when he was found)?

Why did the Emperor let the heresy happen?

And so on.

The story is absolutely ridiculous, but it's fundamentally fun and engaging because it's simple enough to be easy to follow while having enough moving parts to be complex out of the sheer number of characters and moving parts. It's engaging but undemanding.