And the fancy sensors on the mech mean little when guerrillas just melt into population after an attack.
The problem is that again, the armor and sensor density don't allow that. The guerrillas aren't going to get the chance. Imagine that you've got, say, the US in Iraq but this time they have 'mech sensors. Magscan means they can pick up anything metal at a kilometer away or more (BT ranges being wonky), so as your guerrillas are sidling towards the compound, they detect you all have guns and sound the alarm. 'Mechs also come equipped with
chemsniffers so if you think you're going to smuggle in explosives or incendiaries with no metal, good luck with that. You also have to bring a hideous number of troops, 30 guys with SRM launchers are going to need three or four turns of pounding on even a
Wasp before it goes down (source: I ran a brief Megamek test just now). That's a lot of guys to throw together and have sneak up, sensors or no. Try to do it with just a squad and you might as well just squish them all yourself because your main effect is going to be the enemy having to hose your men off the foot of the
Wasp.
As
@Knowledgeispower says, guerrillas can try to target collaborators. However that's not particularly useful in BT either. We see pretty consistently that the ruling powers simply cut out the local government entirely and use their own people. Com* f'rex pretty much always uses a walled compound studded with sensors with only Com* employees allowed inside (past the lobby), and said employees live inside as well so in a pinch, they can just button down and wait it out. In the second Gray Death Legion book the Dracs took over Verthandi and abused the locals, but for their actual mining operation they brought in their own people, surrounded the mine with fences, 'mechs, and sensors, and used their own DropShips to haul the goods out so no native could get within miles of it. The same was done with their walled city, there was simply no native crowd to melt into.