Here are some thoughts.
The POS business is littered with devices that were designed for purpose by giant companies like NCR and IBM. Their primary customers were very large corporate retailers, the cost per unit and for development was prohibitively high and because these were giant corporate IT projects the results were awful for the end users.
At the lower end the retailers got those really basic electronic cash registers (ECRs)with lots of buttons and LCD displays showing numbers only. These were out of Japan mostly and usability was not a priority.
The biggest issue with these ECRs was that they were trying to solve too many end-user requirements, so they all sucked for all but the most simple transactions.
With the PC a bunch of DOS and then windows based retail POs systems emerged - mostly awful usability due to Windows and DOS. The best did made it very efficient for a particular industry. E.g. for service stations their systems were integrated with the pumps in the forecourt and a back office system, all of which likely linked to the oil company. But it all stalled for years until the cloud and iOS based systems came along.
Right now we are in a moment of rapid change, with the end-user experience becoming hugely better for both the customer and the retail assistant. Still plenty to come so watch this space.
(Source 4 years in Mobil Oil, 2 doing payments and POS projects, 2 in the field, and I'm an investor in Vend)