I swap them in regularly in kits like Baron/Sedona, and the Jr Gent type kits. Depending on the kit, you occasionally have to make some slight mods.
For example, on the Baron/Sedona, the tip doesn't protrude quite enough, so you sandpaper off a bit of the nib (thread end). Takes less than a minute - no big deal. There's a video I think somewhere of someone doing this.
I also recall one kit (I think it was a Jr Gent non-postable, but not positive), where I had to drill out the nib, as there apparently the G2 Pilot refill is just a hair wider than stock. I encountered that issue with every kit of that type. Also wasn't that big a deal and only took a couple of minutes. I know I've done other in the Jr. Gent family with no issue - it was just the non-postable batch I had that needed this.
I can't recall a rollerball kit where I couldn't make a G2 Pilot work.