My Uber use is limited to the WDW / Orlando area, so I'm no expert.....but I thought their use was limited to point A to point B, no stops in between? As in if I was going from MCO to a resort, and wanted to make stop on the way, I'd have to book Uber from MCO, to the stop point. Then book a second Uber from the stop point to my resort.
If you can schedule a stop at a grocery along the way w/ Uber, well that sounds very convenient!
Worst case you just have the driver drop you off and call another while checking out. Orlando is crawling with Uber and Lyft, another will be by in under 5 minutes.
Its true that they won't sit around waiting like a taxi would as there is no "meter".
I have had some drivers let me do a quick stop at a convenience store before though.
We stopped at at Walgreen for a late night grocery pick up one time. It's doesn't have as much as a grocery store but you can get eggs, bread, juice, snacks etc