Heavy Cav has needed a nerf for a while. On NA at least, the 5 riding armored horses are very common.
The main problem with heavy cav is that they're nearly unkillable as melee, catas for instance can get stopped in the middle of a clump of melee and ride off multiple times (this is also thanks to the ridiculous forcefield on the elite cav shield).
In my opinion the heavy horses should be mostly immune to ranged, arbs or throwing lances should still take them down but your standard archer scrub with a hornbow shouldn't be able to do much to them - but if they get stopped in melee they should die. I think this is the ideal balance because it gives people a good reason to ride them (avoid being dehorsed by ranged), but you still need to be careful to do well.
Currently though it takes me, a melee with 8 ps and a danish probably 10+ hits to kill a cata, and if the rider is looking to the side and holding up an elite cav shield even overheads from directly behind get blocked.
I think most players will agree with this, the main problem is how to implement it. The horses still need to be tough enough to give them the desired archer immunity, but also need to be killable by melee players. badplayer i think suggested a multiplier, maybe horse HP could be increased and armor decreased (since cut is much more prevalent among melee players, pierce more common among ranged)
I wouldn't even really call this a nerf, the toughness vs melee has been out of control for a long time and even heavy cav players agree.