The GMAT itself is super difficult, so it should be no surprise the GMAT club tests
are. In my experience, those prepping for the GMAT are often slightly obsessed (if one can be slightly obsessed) with finding extremely difficult math questions on which to practice. Some of these math questions can even be un-test-like, i.e. harder and unlike the questions you would actually see on the real test.
Anyhoo, GMAT Club's tests are both difficult and test-like -- so that's a good start.
I wrote this piece about how odd the GMAT really is. Check it out