The DME buses have lifts, but park/resort buses with lifts are very rare at this point. The park/resort buses with lifts may be entirely phased out - we haven't seen any on trips for at least 2 years.
They were being replaced with buses with ramps, which can 'kneel' to make the ramp less steep.
ECVs and wheelchairs should always back into lifts because that puts the majority of the weight close to the lift attachment instead of over the part of the lift that isn't attached to the bus. (Power or manual wheelchair).
The first buses with ramps had relatively steep ramps, which might have been steeper than recommended for some ECVs and power wheelchairs. From what I heard, that plus that there were still some buses with lifts, was why drivers were asking people to back on with ECVs and power wheelchairs. As far as I know, they have not asked people to back onto the park/resort buses for quite a few years.
That said, if you can back accurately, it's a lot less maneuvering to back on - you back on, make a small turn and you are in place. I actually back DD's power wheelchair into our own van because it takes less space and time.