The hardest things are produce and by-weight items. Even self-checkout doesn't handle that well a lot of the time.
They work like this: Scan the barcode on the produce if it has one (or pick it from a picture-list, or search by name, or just key in the 4-digit PLU if you're cool like that), put it on the scale, and it spits out a barcode label that identifies the product and the weight.
At self-checkout, one just scans the generated barcode label and puts the produce in the bagging area like any other item.
Unlike the self-checkout kiosks themselves: There's never any wait to use these machines, so it's an easy process that doesn't involve making other people wait.
This process would work the same, I think, for applications where portable scanners are in-use.
And it has several problems as well with respect to loss (e.g. adding more items to the bag). To deal with that, you'd have to weigh it or count again to verify.
> so you might as well get that done in advance
1. The PLUs are posted on the price signs in the produce section, so it's easy to remember them accurately when I weigh stuff. Entering the 4-digit PLU for starfruit is faster than navigating a list of pictures or searching by name, and is easy when I just saw it a moment ago: Short-term memory is a blessing and a curse, and this process makes good use of it. (I probably won't remember the PLU by the time I finish shopping and reach the self-checkout, except for bananas: That PLU has made it all the way to long-term memory and is 4011.)
2. Stress. The whole time I'm weighing out my produce at one of these fancy scales (I have to weigh it eventually anyway), I am at ease and am free to work at whatever pace I wish. This is not necessarily the case at self-checkout when there is a line of people behind me who simply want me to get done and be out of their way, so they can be done and out of there themselves.
3. The self-checkout line moves faster when my produce is already weighed. This offers me do direct advantage. But unlike some people, I am not a sociopath: Given the choice to make a thing easier or harder for someone else, at about equal cost to me, I try to do the thing that makes it easier for them. So I prefer that the line move faster, even if that doesn't actually save me any of my own time.
tl;Dr, I could behave more like crabs in a bucket, but instead I choose a more-orderly path and weigh my produce in advance.