They are very tight in my experience. Once I had to pay to put my bag in the hold. The bag was the right size to fit into the size gauge, but I had overfilled it and the front was bulging a bit which meant to get it in the size gauge you had to push it in with a bit of force. It went in, but only by pushing quite hard to squeeze it in. This was enough for them to reject it from the cabin and charge me about £60 or something to check it in.
But as has been said before, they make their money on ‘upsells’ wether you want to be upsold or not.
I try to avoid the low cost carriers as much as possible. If you book early enough you can usually get a proper scheduled carrier ticket for a competitive price, or not much more, and i’m happy to pay a small premium to fly a proper scheduled carrier. However unfortunately alot of the proper scheduled carriers seem to be moving more to the LCC models.