Its been a thing for a long time. Rohypnol's success definitely revived its popularity as a "troll build", and I wouldn't say its OP at all. The one problem it faces is that you only need 7 str, and can amass 10 riding skill, paired with an arabian warhorse or a courser is frustrating to deal with for most people.
I don't think it needs to be changed. They can start shooting from a rouncey at level 9 for full piercing damage, while a regular horse archer can't really accomplish much of any damage until his 20's. We have to understand though, that they're crippled by the slow reload, and pretty much any... ability... to do anything else. Horse archers are forced into balanced builds - If xbow were forced into a more balanced build, people might complain less, but please god don't ever make the light crossbow require 12-15 STR.
Like Meow said earlier, pick em off, they have 42 hp at level 30, and usually light armor. Not only do they get the velocity dmg bonus, but you do too when they're coming in your direction.