Even worse than an automobile transmission that has usually has synchronizers to make gear engagement smooth.
I assume JD "unsynchronized sliding gear" is the same as the MT Yanmars. Assuming it is the same, it means a full stop before any gear change. (Or else horrible expensive-sounding grinding). It can't be shifted in motion. Or even standing, after the inertia of a mower etc gets the gears spinning.
Some people might say you can double clutch it but that assumes constant speed gliding along by inertia on asphalt, not lurching over native ground.
Powershift is vastly better. Just shove the gear lever into the intended gear and it takes care of the rest. Clutching is a good idea to ease stresses but it isn't essential.