Is this anecdote or do you have some kind of numbers?
Each time this comes up (the idea that the sleeper is super-unreliable) I go and look at
https://www.recenttraintimes.co.uk.
In the past the argument has often been about sleeper vs flying, and I've gone and had a look at really it's just as likely that your flight will be cancelled or delayed.
Anyway, you can look at the last 4 months of data for the Fort William to Euston sleeper
here.
I've chosen that one as it seems the most vulnerable to disruption - the final arrival time is liable to pick up delays from any of the three highland portions.
That's seen one cancellation within the past 4 months. It's been more than 5 mins late 6 times (out of 72 journeys). Only two of those more than an hour late.
Or the Edinburgh to Euston one
here.
There we can see that yesterday's was one of two cancellations in the past four months. Aside from that, there are only two days it arrived late, and the worst of those was by 18 minutes.
I don't think performance is as bad as is often made out. If you choose to travel on the sleeper, then in the vast majority of cases you'll arrive on time without significant problems. Maybe in one in ten journeys you'll experience a delay but it mostly won't be a dramatic one. The chance of your train being cancelled outright is probably in the order of 1 in 100.
I don't want to seem like I'm saying the mk5s are great - I agree that there are lots of shortcomings in the design & build quality and they clearly aren't as robust as what they replaced - just that the day to day performance isn't quite as dreadful as might appear to people just reading this thread.