Maybe this is common and my experience is at issue, but I found the grid design with the "workers" on top to be really interesting.
My most direct experience was with a tape robot. This wasn't a run-of-the-mill backup library for a small DC, but a huge room on half of a floor of the datacenter that had a 6 axis[0] "bigger than an average adult man" one-arm robot that is usually seen in an automotive factory. It moved on a track the length of the room, grabbed tapes from a library and inserted it into drives; all of which were attached to a mainframe.
This was done with so few sensors, that the tape robot couldn't tell if it successfully grabbed a tape or missed it; indication of failure wasn't realized until the drive reported no tape present when the operation completed.
When it missed, the tape often ended up in the track where it was, sometimes, destroyed or could cause other forms of major malfunction.
Dealing with products of varying sizes, sensors to detect the successful retrieval of a product seem like they'd be necessary no matter how things are setup, but by designing it with the robots on top, a failed grab, at worst, results in the product remaining somewhere near where it is, or in the wrong bin -- but not stuck in the way of the track.
Coupled with sensors to detect "when the product was 'lost'" or where the product was failed to be retrieved from, the system could attempt to "retry the operation", avoiding operational shut-down/having to rely on the skill of operational staff to identify a failure condition early enough to prevent problems.[1]
[0] I think, not positive
[1] There's a study out there regarding factory automation that appeared shortly after Elon Musk gave up on "fully automated production" that talks about how bad people are at responding to "rare conditions" in an automated factory, resulting in costs that often outweigh the benefits of removing people from the process -- this sort of design appears to try to address some of the major things.