Camel archers are certainly not bad. 15% inherent withdrawal chance + no resource requirements. In MP, no resource bonus is more important because it getting your strategic resources pillaged won't hinder your war effort significantly. In SP, it is only relevant where you don't have at least one of Horses or Iron (happens sometimes). Camel archers are the best contemporary killers of longbows in cities.
All UU's are useful in some situations, even the weak ones. Heck even ballista elephants are useful if you are attacking a stack near your city from a bunch of mounted units and siege escorted by pikemen. Your ballistas will allow you to selectively kill the mounted units which allows your other units to mop up the low strength siege and pikemen. Regular elephants would get creamed by the pikemen if they attacked the stack. To make the ballista stronger, they should have made it resourceless. This would allow the Khmer to always be able to build them.
Another weak UU in BTS - Cossack. They should have made these cheaper than regular cavalry to boost them - 100h instead of 120h.
All UU's are useful in some situations, even the weak ones. Heck even ballista elephants are useful if you are attacking a stack near your city from a bunch of mounted units and siege escorted by pikemen. Your ballistas will allow you to selectively kill the mounted units which allows your other units to mop up the low strength siege and pikemen. Regular elephants would get creamed by the pikemen if they attacked the stack. To make the ballista stronger, they should have made it resourceless. This would allow the Khmer to always be able to build them.
Another weak UU in BTS - Cossack. They should have made these cheaper than regular cavalry to boost them - 100h instead of 120h.