Zombies riding horses have great difficulty walking downhill to reach the player

Create a new superflat world and build a 5x5 platform on the ground. Summon a horse in the center of the platform with the datatag:

{Tame:1b,Passengers:[{id:"zombie"}]}

Stand off the platform, and enter gamemode 2. The zombie will try to ride the horse to you, but will get stuck on the edge of the raised platform.