I don't know those station specifically, but I often leave my bike chained up at King's X while I'm away on holiday.
They do come around doing "sweeps", where they tag all bikes, then removing the bikes that haven't been detagged by the owners after a couple of weeks. They're then stored for another short while, and finally sold on. So, apart from theft, that's what to watch out for. Look out for notices to this effect, check the operator's (or NR's website) and, best of all, ask another cyclist who uses the racks.