Yes, fine for multiplayer, which changes a number of things of course (e.g., quechuas aren't so hot in multiplayer). But even then if someone fortified a couple jags on a forest near my capital (I would've chopped the ones RIGHT beside my capital asap) I would just leave them there until they decided to move. I don't see how it ends my game right away. If they move to pillage, I take them out. If they don't, they sit there until I can deal with them. I don't see how that ability makes them extremely powerful.
And frankly, woodsman 2 units might be able to race through wood/jungle to get to my capital, but they'll have a helluva time taking that capital relative to many other units.
Same goes with single player. Your only shot is being able to overwhelm the AI by racing your jags to the destination (which assumes a large amount of jungle/wood between you and them which is not always the case). However, you can overwhelm the AI with pretty much any unit.
The jag is better than I once thought, but nothing outstanding imo, and giving it strength 6 wouldn't make it an overpowered unit, especially not relative to some of the other UUs that are available.