I don't know how Stobart Air compare to others in regard to delay / cancellation due to technical issues, but servicing is prescribed and mandatory regardless who carries it out.
Having skilled engineers on standby and an inventory of parts is incredibly expensive (though less so if you own the airport) which is why it is often contracted out to specialists.
The reliability of aircraft type is a big factor. I have read a number of posts suggesting that the E jets are not the best in that regard, I wouldn't know. Certainly, the last thing Stobart need is a reputation for poor reliability, so they should take all reasonable steps to maintain a high standard.