Quant is rough, IMO. Aside from memorization of pension accting and intercorporate investmentments I didn't find any particular topic that difficult. Howeever, the challenge lies in the vastness of information and time it takes to get a firm grasp of all the concepts, in addition to questions posed in vignette format which adds another layer of difficulty. Given 5-6 months of consistent study you'll have reasonable shot at passing. I put in about three solid months and feel 50-50 on chances of passing, had I started earlier I'd feel quite confident. Put in the time, don't get bogged down, and do all CFAI EOC questions, several time for key areas.
I found the following difficult:
-Time series analysis in quant (only b/c I didn't feel it was worth memorizing the vast amount of info for at most 6 questions on the exam!)
-Exchange rate topics in economics (esp. triangular arbitrage)
Everything else wasn't that difficult after you put in enough time. The two topics I spent the most time on though were pensions and intercorp investments in FRA just b/c FRA was so heavily weighted in the exam and you were sure to see these topics on the exam.