I hear the calling of voices in need of Guidance! As the Oracle of AoW, it is my duty to answer!
Ok, razing a structure, any structure, not just teleporters is a matter of relative strength. Have you ever noticed that sometimes you can blow things up with forces that a moderate gust of wind would knock over senseless while at other times you bring everyone and their dog and it's still no go until you get that lv 20+ hero over to bash things?
The explanation is that the difficulty of razing structures is a combination of an inherent toughness factor of the structure and the strength of the guardian units (which are preset randomly when a game is started on the map). I remember Warlock saying something to the effect that he made razing teleporters (and maybe other structures as well, I don't remember well) harder by raising the structural toughness factor. It was in order to prevent wholesale scorched earth tactics with too small forces.
In general, if the guardian units of the structure are weak, it takes very little to reduce it to rubble, but if they are strong, it can take very tough forces indeed. I remember being able to raze teleporters in one campaign scenario (the river valley on the dark elf route) with a silver or gold medal wyvern rider, but not with an unmedaled one. The guardians were six spirit puppets. On the other hand, one teleporter with a syron took a whole lot more, but I had my leader in that stack, so it was a breeze.
Another indicator of relative difficulty was a death node. I've put a death node guarded by spiders out of commission with a half-stack of fairly weak lv2 and lv3 units, but one with a black dragon had a gold medal gold dragon, a titan and two paladins huffing and puffing to no effect until I got a mid level hero (8 or 9) in there to help. Only at that time did it blow up.
So, in conclusion, if you can't get a teleporter or a node or something else busted with what you consider relatively strong forces, be prepared to face one big bad nasty when you finally put it to torch. I've had to run a fair distance to get away from an enraged syron that came out of a teleporter, and those things are tenacious. Guardians that spawn from a razed structure can never be recruited by those who razed it, no matter the race relations. Others can pick them up, though, so if you've got allies of compatible alignment and/or relations (with the marauders) nearby, they can recruit troops cheaply.
Edi