Hard GMAT Practice Qs Only?

I've gone through the MGMAT WT book and finished all the contained practiced questions, and I plan to do the same for the MGMAT NP book.

I'm considering using only the "Hard Quant" GMAT Toolkit iPhone App questions and the GMAT Club Challenges for quant practice.

Would this be a mistake? Would I not get comprehensive practice, because of a lack of 'easier' topic questions omitted 'harder' problem sets?

Thanks in advance.