I read a lot of praise for a PRB for various things.... magical penetration, amazing trajectories, unimaginable accuracy, and more.
It's a marble of lead. It works. It's what could be produced when firearms were in that phase of development. They went away when projectiles developed further....and for a reason.
Regardless, they are effective, but note we have a few anecdotes of balls recovered from deer shot at high-percentage angles and not long distances, and the balls stayed in the deer.
That is one of the reasons that, when I went to a custom rifle, I went up to a 62, shooting a 0.600 ball. They aren't magic either, but there's a LOT more mass in a .600 ball than there is a 0.490. The 490's run in the 180-ish range. The 600's are 320+. That's nearly double.
Just my view on it, but in a lead ball projectile, "there's no replacement for displacement."
Small calibers can work, sure. CAN.
Optimal? I don't think so, personally. Advisable? I would say only for experienced hunters and solid/capable flintlock shooters. Just my opinion.