There IS actually a train option depending on your time constraints and flexibility, the Amtrak 97 Silver Meteor train picks up daily at Kissimmee Station (nearest station to WDW) at 1:26pm and arrives at Miami Station at around 6:30pm assuming there are no delays. You can take the train to Miami the evening prior to your travel day and stay at a Miami airport hotel that night.
On a randomly selected day in May (the 19th), the price for a one-way ticket for one person from a Disney Resort to the Miami International Airport is $30 non-refundable. For $6 more, you can get a fully refundable ticket (if canceled at least 3 hours prior to the trip).
While I've never actually ridden on a Red Coach bus, I have purchased refundable tickets 2 or 3 times. Each of those trips ended up getting canceled, so I didn't go. I can say that getting refunds from them was super easy. There were no hoops to jump through and I didn't even need to call anyone. It was just entering the confirmation codes and a few clicks.
I know that not the same as actual riding. But, I figure that if it's easy to get a refund, the company is probably reasonably well-run.
Red Coach USA did just add a new route between Orlando and Atlanta. It supposition, but I'd say if they're adding routes, that's another data point that they reasonably well-run.
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