I'd say the two hardest are actually the single-season team and the pitching and defense. The $$ caps for each of those are brutal considering the restrictions.
At $70 mil, it'll be tough stashing more than one or two $1 mil players on the bench. Can't waste money at that cap. Finding a single team with 6-7 solid WIS players, yikes.
And in the P/D league, defense is so darn expensive in this iteration of the SIM, every guy with any kind of hitting ability will cost you at least $5 mil. Kudos to schwarze for a $90 mil league concept where the Pedro, Maddux, Joss rotation might actually be too pricey.