Trolls and other destroyers appear to have some problems with destroying doors while standing on certain positions.
I made an experiment with different quality doors. One door was destroyed by three trolls to compare effects of numbers.
Trolls are these green monsters, they are running to the right side of the screen. Doors are gray objects in the middle, and there is one exit doors in the right down corner.
The doors differ in quality, bottom door is well-crafted, one up is finely-crafted, one up is superior, one up is one up is exceptional, next one is masterful, and the top-most (with three trolls) is
finely-crafted superior, because that was my most common quality at the moment.
After releasing trolls that looked like this:
And this is failed attempt: trolls stood too close to the doors. They were unable to do anything. Something was wrong with their pathing. Doors in the middle were unlocked. The exit doors were locked, that might be the reason:
Another attempt: this time exit doors are unlocked. Trolls stand in the right spots. Now they will destroy the doors.
And that's after some time. Three trolls are clearly better than one troll. Door quality seem to not matter, in fact the first door to fail was masterful, and the last one was well-crafted. But the other three doors were failing in seemingly random succession. The time between the first and the last destruction of door mangled by a single troll was very short. In comparison the door mangled by three trolls was down much faster.
My conclusion was that trolls need to stand in certain spot to be effective, and that item quality doesn't matter. But number of trolls matter.
I have observed on many occasions that destroyers can sometimes stand in the wrong spot, and try to mangle an object. In at least one case they succeeded, after many days. Normally it takes 1-1.5 days for trolls to bring down the doors, depending on their number it may be less. Trolls sometimes don't get stuck forever, they are crowding and for a moment one of them can move to the right spot just by chance. I think this may be responsible for their success in seemingly hopeless situation. But it need more testing.
Anyway, if destroyers are too close they may be unable to destroy the object, but they don't know this.
I have made experiments witch hatches too. When trolls are standing right under unlocked hatch they can destroy it if they can path to it, i.e. they have a ramp or stairs. Not only that, they can destroy doors which are one z-level up, on the same level as the hatch and next to it, without moving to that level. Apparently they reach through the hatch.
EDIT: the topmost door quality was superior. Memory fails, but notes never do.