A typical ECV is in the 150 lb or less range, so same or less than an adult. Not a problem weight-wise since they will take up the space of multiple adults. And this thread has established that there will be a separate loading zone to allow those on ECVs, wheelchairs and with other issues to load safely. Loading itself is drive straight on, pretty basic. Similar to loading into an elevator [and given how small some WDW elevators are, especially at the resorts, you might have more clearance with the gondola ;-) ].
SW