Depends where you are in development in that particular game. As I understand it, that benefit only works for you personally, not your vassals. I held off feudalizing until I formed Russia to benefit, but not sure that ability actually came into play at all, think I had stone hillforts on all mine as it was (partly due to losing a lot of the lower-end demesne counties to gavelkind shortly before...). Well before the time I formed the Slavic Union, pretty much every tribal ruler had feudalized so there weren't really any tribal holdings left for me to use it on between. And insta-de jure on Pomerania and some of the others in contested territory is nice IMO, even if it meant losing de jure on the Russia ones for 100 years, they were mostly uncontested hinterland by that point and safely re-drifted.