Yup, JHI were balanced in a 1.0 environment when the shield bug was still in effect. This allowed them to beat DFK's even with inferior stats. Now that the bug is fixed in 1.02, JHI's stats aren't good enough anymore. They may have good availability but that's not the point. JHI are suppose to fight at least equally with the best European swordsmen.

Against short polearm units like D[N/P/E]K's however, JHI's still perform satisfactorily due to the absurdly slow attack speed of said units. I guess you can say it's two wrongs making it right.

Bob the Insane ran some tests regarding the VG's shield. It turns out it has zero effect on missile defense. We don't know about it's effect in melee though. Here's the thread for reference:
https://forums.totalwar.org/vb/showthread.php?t=83562