The typical, traditional, external door originally had a simple surface mounted deadbolt lock with probably only one or two levers and/or a nightlatch: a “Yale”. (And when I was a lad the door was only locked if you went away for more than a couple of days; or, at least, that’s my dim but rosy memory.)
Then most doors migrated to a latch lock — the “Yale” — and a mortice deadlock with 3, 4 or 5 levers. (Mortised means that it’s inside a hole — the mortice — in the door, rather than being surface mounted.)
Then the curse of the uPVC door and the multi-point lock (MPL) began to appear. You guess correctly that I don’t like them. They look impressive: operate the handle and all manner of impressive-looking bolts, hook-bolts, rollers, mushroom spring out and lock your door. Of course, operate the handle the other way and they all unbolt again, so a key is fitted that locks the handle preventing unbolting. But’s that’s the point. All those locks have a single point of insecurity and a single point of failure: the key and the gearbox.
So if ever your MPL starts to behave differently — making different noises or getting more difficult to operate — do get it seen to. The key should never need forcing; normally it’s the handle that does the hard work and the key gently locks the handle. If an MPL fails in the locked state with you outside or inside, it’s the devil’s own job, and a brutal one at that, to get the door open again. And if you have the worst of all worlds — a multi-point lock in a wooden door — then it’s a difficult, brutal and destructive job to get it open if it fails locked.
Actually the key often does one more job. It sets one more deadbolt, the one right in the middle of the strip. So if you’re getting a multi-point lock, make sure that there is a key-operated deadbolt because it does make the whole thing slightly more secure. And make sure that it operates easily and smoothly. If it requires force the day it’s fitted, it can only get worse. And check the guarantee and keep it, and the installer’s and the manufacturer’s details. And do, please, pass all that on to any new occupants.