Totally agree; try an ECV at home first before making a choice not based on actual knowledge of how you can or can't handle it.
As far as no breaks; it's not like you are driving a car at 60 m/h.
Some normal logical sense gets you a long way. First of all you can use different speed options. The slowest will only let you "accelerate" from standing still to walking very very slow speed. The fastest will let you accelerate from nothing to the max. of a hand full of miles an ECV can do. So in crowded areas, you simply don't put the ECV in the fastest option, but for instance if having 5 options at 3 or 2. Next is practice. It's pretty easy to understand how an ECV behaves and how much distance you need before letting go of your throttle will result in standing still. Keep that distance plus some what extra as the norm. Some folks are arrogant or unknowing (very distinct difference between the 2 groups, but results are the same) and will jump right in front of you. It will take some extra energy as you are not used to it, but being aware of this and being on the look out really is the best medicine against this combined with that keeping distance, not travelling at topspeed etc. More you simply can not do.
If you can use it safely around Walmart you will have a good idea of how safe it might be to use at WDW. Practice esp. at getting around in a small space, making a small turn, backing up etc. Those things can use practice and even just knowing that you have done it succesfully before can make the difference in a crowded WDW. Heck, take a few friends along and when it goes well; ask them to surprise "jump out" on you. For instance right at a corner or whatever. OK, they'll be bigger than a kid but also old enough to safe themselves if you were to get flustered. Again; practice is a huge help as is knowledge that you can deal with surprise popping ups.
But most of all; do not think for others. Whether that is what other guests might think of you using an ECV or whether it not being fun for grandsons to push a wheelchair. Not only is it easy to think stuff that is not the reality, it can easily make you go for an option out of wanting to please someone else instead of looking at your own needs. If you are worried about things within your group; be honest and talk it over. Most of the times it will be worries not worth it. Getting it out in the open gives all a chance to go for the best option for all involved and nobody having a hidden sentiment about how the other might miss out or whatever because of them.