Considering hitting the shield does not cancel the couch lance (as it did in Warband), and hitting the horse does not necessarily dismount the horse, if anything it's the archer at a disadvantage as in order to survive they have to land a hit and even if they do the rider is probably still alive unless it's a high dmg weapon headshot such as xbow. With the flick inaccuracy, holding shot inaccuracy, reload time if you're caught at in opportune time, and generally high manoeuvrability of cav at very high speeds, it's not set in stone at all who wins such an outcome.
That's assuming the archer is aware and it's a solo cav, which is just one facet of couches vs archers and not even the ideal scenario (you should be couching them when they are pressured, pushed, focused on elsewhere etc). Pretending archers don't get couched is silly. It happens constantly, in both games.
It feels less advantageous for cav because often many troops have 3 lives whilst cav have just 2 so the risk doesn't feel worth it, but that's because respawning is a bad mechanic for competitive, not because couches are bad vs shieldless archers.