Horse crossbow is a somewhat better class.
That said, extreme 7/30 builds are trash and I never did such builds. They were popular on US servers for a while (US cavalry fad) because the US servers never had such a large contingent of good archers and crossbowmen on foot, so they could indeed ride around with impunity. However a 7/30 HX will get oneshot by either other HX, foot crossbows, oneshot or twoshot by archers, oneshot by throwing / etc. In my mind the 12+/24 builds are better, because 4 HA is completely enough for reasonable aim, and you can have decent armour and some IF to survive ranged fire which will be coming your way.
Before I respecced at level 29 I had nearly 2:1 K/D with my HX, and if I had a bit more practice (haven't been playing in ages) or more gold (so I could ride an eg. +3 destrier instead of +3 rouncey) it would've certainly been better. So it is quite effective, perhaps not as gamechanging as melee is (bar shooting melee heroes in smaller-scale maps and "cleaning up" in close match endings) but very effective. Granted, I had +3 steel bolts and +3 light crossbow, too. (Well, I was the first HX on EU1, in the time when archers were automatic sniper crossbows of doom.)
The counter to mounted ranged has always been foot ranged (and that counter is now weaker and less popular and much less effective if the HX doesn't care about upkeep and uses a panzerhorse). I tried a HA STF yesterday (with cheap gear, it would certainly be more effective with tatars or bodkins and +3s). It's useful and does do some kills, however it does to me seem it's a somewhat weaker class. It's also a noticeably more expensive class to maintain, which is just unfair, and has to spend all of its points to have a good build by level 30.
The price of bows (especially the medium/lower tier bows) and arrows needs to go down. Other than that, the class itself (HA) needs to boost because you CAN score kills and do something (I tried with nomad bow + barbed arrows and it sortof worked with that, preety sure with loomed khergit and a better bow it'd be much better), but rather the HX needs some sort of str-related requirement to both avoid the 30 agi builds and be forced to spend some extra points (eg. 3-4 points) to force it to make sacrifices if it wants ironflesh / powerstrike.
Alternatively, leave the HX as it is and boost archery a slight bit, at least in their anti-horse capabilities which are now pitifully weak. As long as a HX can afford to ride a destrier it is a nightmare to down it. Since upkeep clearly doesn't prevent players from using such gear, well, you get the situation there aren't enough counters. Furthermore, there often aren't enough players online to have a sufficient amount of ranged (as you see, ranged is quite essential in the game) and HXs/HAs become a gamebreaker.