I can't imagine an effective libertarian militia being able to engage in proper guerilla warfare. It seems incompatible with already proven strategies such as People's War or fundamentalist insurgencies in MENA. The idealogical core needed to fuel it isn't nearly as strong.
Slop youtube and twitter christbro yeschad-posting type medieval apologia and romanticism are absurd to anyone with the slightest bit of common sense but by far the most annoying thing they do in my opinion is to base their entire evaluation of the era inside a small timeframe, usually somewhere around 1300, but sometimes not even a specific and consistent period at all, and then launch all their stupid blanket statements from there, while conveniently ignoring the all-too-common times of crisis, starvation, plague and warfare that continuously take turns with those little islands of prosperity throughout the medieval era. Despite what some might think modernity is actually incredibly stable; there is no more endemic warfare, and there isn't and most likely there won't be any world-ending pandemics or famines, and the only crisis which could realistically happen in our current level of development is war. Just this alone for sure makes up for everything else.