Yeah I agree, having uncapped SF seems like a mistake to me.

Honestly I never liked the idea of SF in a grand strategy game, this is closer to what I prefer which allows you to represent specialized branches of the military such as the US Marine Corps. So I think the mod is on the right track, but with that said I agree I think they are too strong as it stands. I don't think it makes sense to balance them by just raising the production cost even further, as production costs don't effect majors and minors equally.

I think a good nerf would be to leave the production costs as is, but also reduce them to 25HP. I wouldn't want the terrain bonuses nerfed because I think game mechanics should feel impactful when playing i.e. you shouldn't have to look at the stat tooltip in combat to confirm whether there is any difference.

Not that I'm saying I want this in the game, but just for purposes of discussion the only way I could see literal special forces make sense in the game is if Paradox were to turn the current special forces into "Branch forces" or "Special Divisions" similar to how they work in this mod, and then introduce actual Special Forces on top of that with extremely limited capacity and mechanics for actual SF operations. For example, a mechanic in which you can covertly naval invade a naval base and set explosives that literally destroy it so it needs to be rebuilt from scratch. I don't think anyone really cares to see stuff like that in the game, I wouldn't be against it personally but it would sort of be like the Operative stuff in that I just would see it as a distraction from the main draw of HOI4 which is Grand Strategy.