IRA also had a neighboring sovereign state (ireland) that was aligned with its objectives, that helps a lot
but still, i'm not sure i fully agree with your prediction. the modern surveillance state is indeed very good at suppressing opposing organisations, but it also takes a lot of manpower, time, effort, and money to keep that surveillance state up and running.
if the federal government weakens over time, be it due to financial difficulties or political problems (infighting, factionalism, corruption) then the surveillance states effectiveness will slowly decline.
and always keep in mind that the march of progress affects both sides - 30 years ago the only 'organisation' opportunities for right wing militia types were basically hillbilly get-togethers in remote rural areas. now there's the internet, with chuds everywhere being able to communicate and exchange information with relative impunity. i can hop on youtube right now and watch dugan ashley step on israeli flags while explaining in great detail how to manufacture homemade explosives.
yes new monitoring and control techniques are constantly applied to the internet, but at the same time new ways to get around that shit also keep popping up.