It's far harder to defeat a barbarian sitting the barbarian hut at higher difficulties. The huts spawn barbarians so fast that you need to produce a specialized force (2 units or more) to take them down. Plus the 50% chance is a hogwash. So instead of creating a military force from nothing, you need to create a military force just to create a little bit more military force.
The UA is far less useful than you might think at higher difficulties. It sucks.
The UU however, could be really great.
There's two reasons I like it anyways:
A) Go honor as first SP and you don't need a hardened force
B) Build two warriors and you no longer have to build military on high levels (which you are otherwise forced to do to survive the inevitable early DOW)
The 50% chance does blow, though, due to inconsistency. IMO they should've eliminated the gold and made it 100% chance. Or better yet, allow you to use 25 gold to bribe them to your side.
Edit: Also as for needing a hardened force, you're sorta doing it wrong. If you fortify on a hill barbarian spawns will ignore your unit. Wait for it to leave and then smash the encampment. Rest a turn if needed. As well, if there is an A.I. next to the encampment, and their frequently is, simply let it attack first, then finish it off.