When I first started reading the mainline Horus Heresy novels with Horus Rising, I had only the most rudimentary knowledge of Warhammer 30k/40k lore. Some of the foreshadowing of those early books was lost on me, because I wasn't more intimately familiar with the setting, but on the other hand, I was able to be surprised by revelations, character arcs, etc. because I didn't already know what was going to happen.
With Nemesis, even my rudimentary knowledge of the setting was enough to "spoil" the ending. Heck, even knowing that there are more books in the series beyond this is enough to spoil the ending. That would be fine if some interesting macro-level developments happened in the background - the macro, world-building parts are often the most interesting anyway - but there wasn't much of that either.
That said, I think this was one of the better-written entries in the series, and I could see it appealing to a wider audience than much of the Horus Heresy does (of note, this book managed to crack the NYT Bestseller list). Focusing on expertly-trained humans rather than superhuman Astartes allows for a sort of "middle ground," where you get to follow characters that are more emotionally relatable (well... some of them), while still getting to see them "do cool stuff."
It was a bit overlong, and not that noteworthy in the grand scheme of the series, but it was well written and enjoyable. Additionally, the last, post-climax chapter has some clever things to say about the setting, written with more subtlety than you often see in licensed fiction.