They will work pretty good as long as you don't try to use it like a brush hog. Brush hogs have blades that swing on impact with a tree or rock. Most of the trail mowers are connected directly to the crankshaft of the engine, and you stand a chance of bending the crank if you hit something hard. They are designed for finish mowing, unless your looking at one I'm not familiar with.
I have seen some ads on tv about a self propelled walk behind mower that was cutting thru 2" trees, and 6' johnson grass. I think the brand was DR