What do you mean by shuttles? If you mean shared-ride vehicles where you may end up riding with other people, those don't exist. That's what Disney's Magical Express is (which is run by Mears).
If you're going to a non-Disney hotel (like the Walt Disney World Swan and Dolphin or the Disney Springs Hotels), you can get a shared-ride trip from Mears. They depart from the ground level on both sides of the airport.
I used to use Mears to get to/from the airport back when I lived in Orlando. I also used them once last year to get from the airport to one of the Disney Springs Hotels. I've never had a bad experience.