I've seen it work only once, but it was amazing and killed 3 battlesuits. It was on atcs, in the middle room. Granger moved part of base there - an egg or two, possibly overmind, two acid tubes and a trapper above door. The trapper was placed in such way that it wasn't visible until you got under it.
A battlesuit entered, and completely stopped. He tried to backpedal, but couldn't. He rotated and tried to go away, but couldn't. All the time a two pipes were pouring acid on him, a granger spitting and dretch bitting his ankles. He melted.
Two next battlesuits died the same way. One of them realized what's going on and started firing on the trapper. Too late.
Description says trapper is effective mostly on ceiling. Place it in bottleneck, where enemy can't see it before it's too late.And where it can't be lucifered from far away, or from around the corner. And trapper needs some backup, tube or two, an egg providing dretches or something. It's not useful on it's own. Highly situational.
By the way, hovel is not useless. It's use is to place it in a bottleneck and use as a free wall. It uses no conscience, so you lose nothing by placing this wall somewhere. It's big, much higher than its model suggests.
Hiding inside a hovel... oh, you romantic.It's asking for trouble, grenade, lucifer.