I had the same experience. I sent in a ton of books earlier this year and just about to a book they were all quite under graded by MCS, in many instances it was not even close. For example I sent in three books that had been cracked out of cgc slabs, a couple of 9.2s, and a 9.4. The 9.4 book was given a grade of VF-, one 9.2 was given a raw grade of FN/VF, and the other a VF. Many other books that I was very confident about grading on came back just way, way too low. A few books here or there I could understand, but it was almost every single book. sigh
I mean I understand being conservative with grading, but this is a little extreme IMO and it was the only part of the selling experience that I had any issues with. So if you are buying there, ya you are likely going to do pretty well on average with actual condition of books vs as advertised condition.
Everything else is handled top notch.