Auto subs work, the sliders do not. I have not fully tested for all positions, but at least for the RB (which is one of the most important positions in terms of fatigue) the sliders absolutely do not work. I have put it on both the 0 end and the 99 end, neither work.
Basically if you put your rb in and run 5-8 straight plays (depending on their fatigue rating and the fatigue slider) they will come out but ONLY AFTER they hit the red zone for fatigue. This is regardless of the slider. What should be happening (and does happen in play now according to what im told) is that when the slider is at 70 or so, the back will come out when he hits the yellow zone, or around 3-6 plays (again depending on sliders). This is a very important issue for people playing RB committe teams like Chicago, NE, Carolina, etc. The only work around is to constantly sub players out yourself which is yet another work around that we the players have had to create to fix this game.