When I put /ride command in repeating command block, it didn't work properly. I only rode the mob when I pressed shift for a brief moment, but then I dismounted from the mob (I basicly telepoted to the mob). On the side note, I had tp command in another repeating command block that would activate unless player had certain rotation nbt data (tp command had these coordinates+rotation: ~ ~ ~ 0 60). Also, command didn't seem to properly work when I dismounted from the mob. I could then move around freely unless I pressed shift again. Then I teleported to the mob again. Hope this feedback helped.