It seems they fill a very niche role, and one or two units is enough for the rare times they are helpful. They're main role being standoff, out of archer range and compel the enemy to advance. Certainly they would be great defending walls or bridges, but offensively?

Yes they can do well on the flanks, or out of ambush. But so can woodsmen and just about everything else.

I've tried custom battles with 8 firearm infantry in a "thin red line"... the sort of tactics that were common from 1700-1900 AD, and this doesn't work. It's possible to beat a similar value AI army on open terrain, buts it's definitely a phyrric victory.