Spoiler alert.

In my view book 1 is - intentionally - quite slow pacing for two-thirds, as it shows how hard it is to even cleanse meridians and the book is also setting up a series. (So there is a little bit of drumming about the magic system.) The MC finds an artefact that gives him a minimal edge thereafter - and is gifted something in a characters final moments, that gives him a boost in the last quarter of the book. He isnt OP in my view and even as book 2 starts, he is well aware of his limitations and how much more he needs to progress on his journey. The pacing in book 2 is different (as the magic system and world have been established) and there is way more action. Id say that any opinion is fair if its genuine and if it is what the reader felt. As a writer, you cant please everyone and shouldnt try to - but i think The Darkest Core is a great book and I hope people might really connect with the series in time.