Maybe just one bad experience for us but we've had and borrowed roof boxes without any problems.
One year we went with a roof bag (as the LR Disco we had didn't have roof bars and I thought we'd save a few quid). We used it for a three week holiday which involved Cheshire to two stops on French campsites then calling in at relatives in the Netherlands on the way back so a few miles and a few packing/unpacking. It did its job but there was quite a bit of wear to straps, zips etc. We changed the car anyway but I wouldn't have taken that roof bag on another similar trip - it didn't look like it would survive the same again.