The age old conundrum of the unit that may or may not be strong in real combat situations, but becomes absolutely gamebreakingly busted when added to videogames, because it's strenghts translate into overwhelming advantages with none of the real life drawbacks it had to endure, usually via game design, bad balancing or games putting said units in unrealistic situations.

Take for example anti-aircraft guns since WW2. Other than the obvious real example of the FlaK88 being turned into an AT gun by the Germans, several others of these become anti-infantry or even anti-armor rapid firing nightmares in war games, because they're put well inside their optimal range and within threatening range of infantry and tanks. Which would usually destroy them from afar. The OTO Melara gun is a good modern example. Italian radar guided 110mm naval gun, was never mounted onto a proper line vehicle that was adopted by any country. But the prototypes, like the OTOMatic, absolutely terrorize every game where they appear, as a hyper accurate, rapid firing, high damage anti-everything gun.

Horse archers are just the ancient ages example of that.