The fact minecarts and horses were governed serverside was what made reliable elevators and other transport mechanisms possible even on multiplayer servers in laggy situations.

Now that they are clientside, things like slimeblock minecart elevators no longer work at all, because the very precise timings on which these mechanisms rely (it is really down to single ticks) REQUIRES the cart/player to be in exact sync with the piston actions.

Please revert this behaviour.