I'm on book 22 out of 54 in the Horus Heresy series, and I can attest that it's very uneven. I've not found any downright BAD books so far, but some have much more interesting stories than others. After a while some of the stories kind of blur together because you can only have so many remarkable characters in legions made up of thousands of nearly identical characters, except for the Primarchs and the occasional original character that's not a space marine.
It's my first dive into Warhammer 40K lore and I figured I'd start at the Horus Heresy because it explains why the galaxy is how it is in the 41st millenium. It's interesting and I don't regret reading it so far, but the epic / memorable moments are very spaced out, you can have entire books where the status quo hasn't moved an inch.
In a way, it's realistic than on a galactic scale, a Legion or some isolated garrisons don't know yet about what is happening on the other side of the galaxy even months or years after, but it really slows the story moving forward.
Considering said heresy happens 3 books in, you'd think by book 22 I'd be starting to see some shit happening near Terra, but nope.
Feels like a super slow Warcraft 3 multiplayer where everyone plays turtle.