Americans have enough indirect fire units, it's just that half of them are of questionable performance (namely mortar halftrack and pack howitzer).
As for the stuart, I'd trade the gimmicky abilities for better base stats so it could snipe infantry time to time and maybe even circle strafe pIVs in the best of circumstances.
USF has plenty of indirect-fire units, but lacks a non-doctrinal barrage unit. Instead they have the pack and M8 which both are designed for attrition, not destroying heavy defensive positions and blobs. Not all indirect units are the same.
As for the stuart, I guess I'm the only one that likes this thing. I just had a match where I got it to vet 3 and it was pretty sweet. It's stun ability is actually good for protecting M36s and have used it successfully as such many times.
It's engine killing ability is pretty niche though and I wouldn't recommend using it unless your enemy can't really deal with the stuart because it has bigger problems. I did use it to effect once when I pincered a Tiger that was trying to run from my jacksons and managed to kill it which was pretty sweet.
If it needs a buff, I think it could use some more health so it can survive one more hit. Getting two shotted by everything makes it very hard to use. Maybe a tad bit more Rof as well.