Everyone says Unit 9 and 10 is in the whole different stage - but why?

So I went through all chap 1 - 10 in BC, and I saw some people saying "Chapter 9 and 10 is just so brutal😭😭😭", especially the one who treats the previous chapters as in "extremely easy" category, but why?

Like for C9 polar coordinates are pretty much a new concept, and it might be hard (who cares about vectors and parametric, they are a common sense). C10, sure, the logic behind series is pretty new and some of it is hard (eg. Lagrange thing), but other than the few hard ones, is it more of like just pure simple logics? Like integration tests or those tests are just some applied calculus in series perspective, and other tests, like ratio test or so makes so much sense.

Anyways, why do people treat C9 and 10 as in like a COMPLETE DIFFERENT LEVEL???