One issue we had with renting directly from the parks themselves is that save Epcot, the WCs were all physically past the gate, and we all know how much fun it is trying to negotiate the mass of humanity just inside especially at park opening, and if you don't know which side guest services is on, you could be pushing through from one end to the other. the other issue was being made to swap from one chair to the 'courtesy' chairs to get up to the monorail. Also, I found no place to get a WC at DTD, and that is a fairly large stretch of real estate.
They do offer multi day packages at $8/day as opposed to the one day price of $10, but that is for manuals only. Both manuals and
ECVs are first come first served. One morning we got there just as the park opened and the guy ahead of me in line shelled out the money for FOUR ECVs. you can use the pass multiple times on the day it is stamped, so if you start the day at MK, in the evening you can trot on over to Epcot , and if a chair is available, you can still get one on that day's pass.
As for ECVs being dangerous if you aren't used to them, I witnessed first hand a woman at Tiki Room literally smash a plastic piece of her
ECV into tiny bits trying to situate herself for the show. she impaled it on the edge of a bench. She also ended up blocking the exit for a good long while, as she could not figure out the controls, so people could not exit at the end of the show.
We have already decided that next time, we are renting one from outside and will have it with us at all times, although I am mobile(for now) and it would make it quicker when it comes to transferring to and from buses and monorail.