DH and I use SS every year when we go to DLR. Based on my email receipts, this year will be the 8th round-trip with SS and so far, we haven't had any unexpected problems with their service.
Waiting:
Yes, we have to wait at the airport for the shuttle and often we circle LAX for passengers. The longest we've ever spent transporting ourselves from curbside at LAX to the Fairfield Inn was 1 hour and 20 minutes, which was caused by a lot of single party passengers that needed to go north of the DLR area - that meant more stops than usual. Usually it's about 45-55 minutes...if I remember correctly.

Returning to the airport, SS has always been punctual. The company provides return time options that I can schedule, which are very conservative, anywhere from 2.5 - 3.5 hours prior to the flight departure. I agree with
@TechGuy that for us, leaving early is preferable for us because no one can predict an accident or problems with TSA security lines.
Crazy Drivers:
One was really fast and I held on tight for that ride, but normally they are unremarkable or a little speedy. Some are pleasant, some are boring, one was really interesting and DH held a conversation with him the entire ride. I thought they'd exchange phone numbers! lol
I think it comes down to expectations and how fast you want to get to the hotel and parks. SS shared ride isn't the fastest way to go, but it's inexpensive and gets you there in a
reasonable amount of time. It's not the way to go if you want to get to your hotel directly with zero delays.