As others have said, it's not so much when you're travelling with them that's the problem, it's when you're not! The new trains are a welcome boost in capacity, and I can imagine that in a couple of years time the overall TPE experience will be better than it was in (say) 2015.
But since 2018, they have had a high frequency of cancellations, particularly in the aftermath of timetable changes: December 19 was the worst, but there were issues both after May 18 and December 18 as well. It'd be wrong to imagine that there were none in-between those pinch points either: their Scarborough service in particular has been bad, but really on all routes they're prone to cancelling services at York in order to maintain core frequency. In turn, the core remains prone to overcrowding as there is such demand on the Leeds-Manchester-Liverpool segment in particular that a minor delay on the periphery can create huge impacts down the line.
The underlying problem is attempting to run mid-to-long distance services from Scotland, the North-East and Yorkshire to Liverpool and Manchester Airport through a heavily congested urban core (between Leeds and Manchester). Unless you have excellent organisation, and some spare capacity, problems will reverberate across the length of that network and the only fixes are cancellations at the edges, or overcrowding in the core.
