One question to ask is - does it have the money back guarantee?
As to the difficulty, the course has to be strong in basics to give you a good framework for the test. It does not really matter how many hard questions it has. That's a less relevant (still relevant) aspect.
Thanks BB for your advice.
They offer money back guarantee for 30 days.
However,I have decided now to take up only courses that has verified reviews like Knewton,Gmatpill
and not try anything new, even if they provide a money back guarantee..