Anything with a lot of tracks. To keep track of all those tracks and learn where and when and why they have to be (without believing in a lot of tracks philosophy). Or, very sparse minimal music that must become also listenable. Music produced by a musician who's read every issue of MIX, Studio Sound and Pro Audio review who criticizes your every choice of gear. Bass player who didn't like his sound because I was using a 1066 and a UA175 for the mic. and the same setup for the DI. I asked him what he would use and he suggested an 1176 to take off the peaks, a Pultec MEQ, an LA2A for compression and last but not least a Pultec EQP, he didn't know anything about mic. pres. so I was safe there. I asked him if he had this stuff in his rack (ha). He had an Ampeg V-4 which was nice and I put a U89 in front of it, which he didn't like (he'd never heard of it). So, I wrote on the tape strip, BSmic. and BSDI when I should have written BSDI and BSmic., I asked him how the DI sounded and he said "flat, muddy and distorted" and I said "oh **** I made a mistake that's your amp" which sounded fine anyway. For 15 minutes that was difficult to mix (I used only the DI, it was easier to push up the 800Hz , no amp fan noise).
Something similar, the producer who wouldn't change a date so I could play a percussion background singer gig was really difficult to mix (I just love to play cowbell). What am I saying? Technical factors have little to do with the difficulties you find in dis biz. Hermes Trimegestis " the universe is mental".