None to bad. You do have to spray wash them periodically (but you have to do that with a side rad too), and the after cooler too, which in my rig requires spraying from inside the bedroom, not a big problem though. Some older rigs have a slobber tube that drips oil out of the engine that can gum up the rad, usually from too much oil during a fill. But knowing how much to put in can reduce that problem.
They are in the way when the rig needs some simple service, but the techs I've had work on mine always seemed to get things done fast enough.
One benefit is that there is less to go wrong with a rear rad...no complicated hydraulic system needed to spin the fans. Just a fan, belt, and a couple pulleys.