It's many years since I dabbled with quake based game level design (rtcw, sof2) and back then I used Worldcraft (now Hammer). From the video, it looks like Trenchbroom is a fair bit more simplistic, what is the benefit of this over Hammer?
It just works. The overall modelling workflow is much more efficient, it's well designed, linked groups is a killer feature, it doesn't crash for no reason and when it does crash you can submit an issue and get it fixed in a day (or fix it yourself, it's FOSS), CSG (e.g. the carve tool) produces sane geometry in a few milliseconds instead of crashing after a 20s freeze.
It also runs smooth and fast whereas hammer stutters all the time and feels like it renders its viewports and UI at 20 fps.
The apparent simplicity is a feature in itself, not a lack of functionality.
From my doom level editing days, this was always the biggest issue; the level editors were so very touchy, and would crash all the time.
Simpler doesn’t always mean less powerful.
It makes making complex shapes so easy that the standard of a good map is way higher now.