We've had multiple discussions with bus drivers over the years about this topic, because we have noticed there can be a wide difference from driver to driver as far as how "accommodating" they will be.
It's our families understanding that they are NOT supposed to drive, pick up, push, pull, or otherwise move the
ECV (or any other mobility device); that is supposed to be done only by the Guest or their caregiver. The only exception to this is supposed to be if during the course of putting the tie-downs on the equipment, they need to shift the equipment, turn the wheel(s), etc.
However, as most of us have seen, they will do more than they are supposed to. I believe it's partly because they can speed the loading/unloading process by "helping", but also because there are far too many Guests who get to the bottom of the bus ramp and just throw up their hands and say "Oh! I can't do that that!" instead of taking the time to *learn* how to drive up on the bus, make that little right turn, and essentially parallel park the ECV. It's not hard to do - but I agree that it can be intimidating the first time.
If you follow the driver's instructions, go *slow* (Turtle mode is our friend here!) and line up as straight as you can before you start up the ramp, it's pretty straightforward. (pun intended! LOL) It's all about control; being able to control the speed and braking of the ECV is the primary skill set required for this. I have noticed that a lot of people get freaked out/startled/scared when they make that right turn at the top of the ramp, and they can feel the rear wheel(s) are still kind of down on the ramp; it feels like you might slip backwards, or you may have felt a tiny "bump" as you turn to the right because of the difference in the floor of the bus and the lowered ramp. But it's safe - I promise you that! - because if it wasn't, Disney wouldn't allow Guests to do it *thousands* of times a day on all of those buses!
The drivers will coach you, if you let them, on exactly what to do - just be honest and upfront with them, and tell them that you need some coaching on how to drive on and off the bus, and park. Believe me, they will be glad to help you learn, because they know that you will be not only faster to load and unload once you are comfortable with the process, but you will also be safer when you are in control of the ECV!