IMHO
It depends partially on the mattress, but I believe the real culprit is the frame. If the futon in question has round metal bars, it doesn't matter if they have 3" or 6" spaces, every bar makes a pressure point, and are incredibly uncomfortable. If you are buying a decent wood framed futon, with slats, that have about a 1" to 4" spacing between the slats, and it folds out into a nice flat surface without a ridge down the middle of the two pieces, then it can be quite comfortable. You can even take another person's suggestion and use a memory foam mattress topper to add extra comfort.
I would honestly buy a well designed wooden framed futon, but never a metal one.