I've taken the sleeper five times between Aberdeen and London and the first three times the Aberdeen leg didn't run. The last time I arrived into Euston 90 minutes late (though that wasn't the sleeper's fault).Frequently fails? It is the most reliable train service running between Scotland and London. Normally arrives on time (or early) at destination, is never overcrowded, never declassified, never terminated at Preston etc.
On the bright side, delay repay means it's quite economical!