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).
On the bright side, delay repay means it's quite economical!
You're definitely incredibly unlucky. I must have used the sleeper atleast 20 times over the last 10 years and only once had a cancellation.
I find it very reliable indeed.