They are indeed intriguing. They also were popular for duelling, the normal duelling pistol side hammer and nipple are in your vision when aiming, its off-centre motion and the exploding cap are distracting to the shooter. Unerhammers were less popular on rifles where you had your off hand and the grip in a vulnerable are, and it could not as easily be fired prone where the hammer might catch on surrounding materials near it. Some came with an optional forearm which made it a bit easier. Target sights and optical sights were also available.
Some pistols had trigger guards that also enclosed the hammer to help keep it from snagging. They also came in breech loaders!
See
https://www.youtube.com/watch?v=QjzrUYgAzAI
And the Ultimate underhammer is a pre-percussion flintlock - how it works without the priming charge falling out, I have no Idea.
When the under hammers came out, duelling was however dying out in the States.
If cleaned and loaded correctly and consistently, they were quite accurate at pistol ranges, a .36 with a conical projectile is essentially a .38, a calibre still in use by police and world military inc. the USA with its metric disguise of 9mm. The 9mm, .36, .357, .38, are essentially all the same diameter, the cases are different for different powder charges and loading/ejection.
The breech loader underhammer was .58 cal. & originally fired a greased minnie-style conical projectile using a paper cartridge. The pistol uses a 40 grain chrge, the Rifle uses 140 grains and kicks a bit.