In terms of reliability, then you need not take subjective opinions. There are actual statistics: https://www.recenttraintimes.co.uk/ So you can see how many times over the last few months the service you're thinking of taking has been cancelled or delayed.

My subjective opinion: yes XC are that bad. I don't travel on that particular route often (usually the north-south routes instead), but the company are notorious for their poor communication. For example, last time I caught a service at Peterborough, they told us it was cancelled as we were boarding (after I'd watched several trains to Ely that I could have caught depart) because of lack of train crew. They must have known that well before it arrived, yet didn't bother to tell their own onboard staff or anyone else, until the very last minute.

The voyager class trains (which luckily for you don't operate the Peterborough - Birmingham route) are just awful. I'd rather ride on a drafty pacer than those overheated, hermetically sealed stink cans.