They're generally fine. The trains they have (known as "Adelantes") have had reliability issues over the last year or so and, as a small outfit with little contingency, that's caused a number of cancellations.
As the frequency of the Sunderland trains isn't that great, they'll generally accept tickets on LNER if your train is cancelled. If you have assistance booked, you'd go and present yourself at the LNER customer service desk (and they're great at both KX and York) who would sort you out.
Looking at their performance figure (services arriving no more than 15min late), it's gone from 71% this time last year to 92%, so a lot of the problems have clearly been resolved.
A lot of the issues rerouting come from GC mostly serving stations that have no other Inter-City services. This certainly isn't true of York.