what problems? only thing I know of is wearing out the unit bearings, mostly because they use wide wheels, and wide wheels would kill them with 35's or 33's I'd bet. Then when you get into larger tires they get heavier too. So it all comes into play, but most of the time guys go 6 or 6.5 and full crank, or 6 inch and do the same, that tears up all the front end stuff if the angles get bad. You can do that on a stock 4x4 crank too high and wear it all out.
The main thing is ppl want to get higher, not many companies now have the ifs lifts bigger then 6.5 and they aint cheap, it comes down to being cheaper to solid axle swap. Plus you can go higher. The solid axle gets ya about 3-4 inches itself, add lift springs and you can go way up there.