I have a Detroit locker in my D44 rear and a Lockright in my D30 front. I've driven on dry and wet pavement (not in 4wd), icy roads, packed and slushy snow, and off highway (usually in 4Hi or 4Lo). The only time I have really had a problem is way off-highway in an off-camber muddy sharp turn going up a hill - with only enough room between the trees midway up the hill for a few inches on each side of the XJ. With front and rear locked, my Cherokee kept sliding sideways.
I have had no problems in any other situation (knock on wood). It does take a little more concentration on the driving, but it is very manageable. With that said, I don't think I would ever loan the vehicle to someone else to drive - it is just that much more difficult to drive that I don't think it would be a good idea to let a novice drive it.
I've never driven a selectable locker, but it would seem to be a better solution for a daily driver - but I've had no problems with mine as a daily driver.
Dale