I also much prefer this scheme because it makes the drop off faster and more frictionless. Courier knocks, hands over the food, and leaves. It takes only moments, which is great - just like getting out of the car with Uber. No fiddling with devices etc. It avoids the problem of the courier having to stand in the doorway awkwardly while I unbag and check all of the food. Now the drop off is quick and I can assign the rating and tip at my leisure. (If there is a food problem they can't really fix it by then anyway)
I had the same concern about "who/what exactly am I reviewing?" However I eventually resigned myself to simply reviewing the overall customer experience. It's up to Postmates to perform quality assurance on each component of the delivery process.
I think the courier has the most effect on overall delivery quality. Bad couriers will miss restaurant mistakes even when obviously printed on the receipt. Good ones will catch mistakes even if they're not, by reviewing the order verbally on pickup with the host or hostess. There is also a central dispatch component of the service, the phone reps who call in orders; they can also make mistakes. I suspect Postmates could increase their accuracy if they could get restaurants to participate, so they could send orders in writing. Granted, there are certain restaurant problems that Postmates cannot catch: like the kitchen makes and boxes the wrong item without realizing it, such that the wrong item is on the receipt and the host doesn't catch it on pickup. This failure mode seems to be fairly rare at professionally run restaurants though.
A good courier can catch nearly all mistakes by comparing the receipt to the order, and by reviewing it with restaurant staff; and courier delivery is the majority of the variable delivery time since restaurant kitchens typically get food out in 20 min - so overall I also think it is appropriate for them to carry the bulk of the review/rating focus. If you rate 3 stars or lower, Postmates also asks you to check what went wrong: missing items, etc. Ultimately, we should as customers report our experience on each delivery and trust that the service will dig in appropriately to whichever component is responsible for quality failures and fix it - that's one of their value-adds. We should not worry that our reviews will be misused or misapplied because that would be assuming bad judgment on the service's part. As a customer I don't want to, and shouldn't have to, think about where the source of mistakes might be. That's where I've mentally ended up on that issue.
I have had quality issues from time to time, that once led me to send an email to their CEO after a string of 1-3 star orders (no response; didn't ask for one), but lately and most of the time things are pretty good and arrive as they should. Postmates CS has always made mistakes right with me, which is why I kept using them even despite the errors. At one point I worried they would suspend my account for fraud, when I was asking for refunds on 30% of my orders, rising eventually to 50% during what was presumably a growth spurt. But I sent CS pictures of the mistakes (which they said was not necessary) and always got the appropriate refund. Mistakes are fairly rare now, and most of my orders are 5 stars, though I imagine their concession rate will be a major area of focus in general going forward.