Their Vet is very high so its especially hard to vet up. But the upcoming back I believe fixed some of the vet requirement so they shouldn't be so difficult to vet up.
JLI are similar to Pathfinders as they are designed to supplement your infantry with long range firepower over being a tanky mainliner. Both are squishy and dont hold up well under concentrated fire.
Atm, they seem like a waste of popcap, having 4x volks and 1x jli is not better than having just 4x volks and saving the popcap for a useful unit. The stealth and infiltration isn't really useful as it doesn't allow you to flank with them and infiltration is not good as they cant really do anything alone anyway. They seem to lose to everything, if both are in green cover.
All of this is just my personal experience, perhaps there is the same problem with pathfinders as I never see them used either. In two months of playing(almost), I have never seen anyone use either unit. Or perhaps the problem is my lack of skill, but I really like the unit but I can never make them work, I also don't want to use the Breakthrough doctrine instead as I don't like the blob encouraging playstyle.