For one, the capacity on buses has been reduced by more than half. That means there will only be a small handful of parties getting on any given bus. Wait times for a bus will likely increase during peak times of the day
Second, you do not want to be in a bus environment with your family right now. It's a higher risk activity and just not worth it considering you can easily just get your own car and thus set your own schedule and not worry about physical distancing/mask wearing/what other people on a bus are/aren't doing.
Disney's Magical Express will be dealing with the same neutering of capacity as well with the same set of concerns.
Renting a car should be a slam dunk for WDW guests right now.