Enjoy the battering ram while it lasts because it is grossly overpowered and likely to get nerfed.
No it isn't. It's a very strong unit for a civ that is only strong in the earliest stages of the game, and so needs a very strong unit. It can only attack cities and doesn't do a lot of damage defending itself. It denies the Huns a melee unit stronger than the Warrior against other melee units until they get iron (or Civil Service, if no iron in their territory). Unlike all other siege units it isn't ranged, and so any defenders preventing it getting close to the city nullify it completely.
I say this having lost a city to two hits from a battering ram in my current game; it had however only just grown to pop 2. Not because the ram is overpowered, but because this is the only way the Huns can compete. Indeed they lost their entire army in that attack, and the victorious ram was sitting around waiting to be shot once the city was razed (granted, a human player would probably keep the city and have the ram safely inside).
The battering ram is one of the most interesting design features of G&K, and its power level is pretty much exactly right for what the Huns are - compared with some of the advantages other new civs get, and it's very far from an overwhelming advantage. And speaking personally I like to play a game where I will take losses in a war - it makes it feel more challenging (which is not necessarily the same as being more challenging - I have no prospect of losing the war against the Huns, I've already teched to a point where they have little chance of beating me, and losing a small, newly-founded city likely cost me less in the long run than the Huns losing their army) - and indeed makes the Huns feel like what they're meant to be: a short-lived threat that can rampage through early cities but is unlikely to actually wipe anyone out.
But the Horse Archer is so strong it can decimate all the enemy units before the battering ram arrives.
The Horse Archer has a fairly low strength, just a high ranged strength, and ranged attacks take longer to kill things. It's very vulnerable to Spearmen and to some extent enemy archery. As someone else noted, it's a 2-range unit that can't move after firing, so the 4 movement doesn't help it defensively. You're also limited in cities/production that early in the game. By the time you can build up the size of force needed to swarm with horse archers and rams, they're already redundant technology.