The issue with RedPower is that if you can do something with it - there is a mod with something that can do it better or in more simple way - with a magic block. Well... some frame machines are exceptions for now.
Treefarm/other farms? Forestry can do it with a magic block - no need to make a control circuit of computer.
Quarry? Well... WayOfTime made a nice frame quarry, Direwolf made a frame quarry too. The are different, but each of them is awesome, but... Well, BuildCraft quarry and miner are far more simple than these frame machines. And with WayOfTime-style quarry you have to use any mod with big chests or enderchests, since there are only a small room for a chest.
Sorting/autocrafting? Logistic pipes... I don't use them (and buildcraft/forestry too), but... well, they are simple, compact and very handy with that "requests" system and other stuff.
Computers. Well, how often did you hear something like "I can't understand FORTH, I'll use computercraft and Lua". RP computer is awesome, but not very "user-friendly".
I really like RP, and I don't use BC, Forestry, computercraft and Logistics pipes, but I think it is an issue - you can't build with RP Blocks something better(or, at least, more simple), than one can code and make a mod.