Most Challenging Poker Variant?
I was reading James McManus' excellent book Cowboys Full where he said that two of the most challenging poker variants are H.O.R.S.E and 2-7 lowball.
I can understand that HORSE would be challenging as it would require mastery of five different variants and being able to adjust between them, but could someone explain to me why lowball would be considered so challenging?
I have also read that 5 card draw is perhaps one of the least challenging variants. I was wondering whether anyone agrees or disagrees with these assessments.
FYI when I tried playing a few hands of PLO, after NLHM, it seemed infinitely more complex to me.