I had a PlusBus ticket for the Glasgow area on Friday, and it didn't go smoothly at any point.
Firstly tried it on a Stagecoach bus, which had no barcode scanner. Driver peered at it for a while but eventually let me on
Next was on a McGills bus (local company bus managed to buy out Arriva so a decent size), scanning the barcode produced an error. Again the driver studied the ticket for a long time, but eventually let me on.
Finally it was on a First bus, and again the barcode failed to scan. The driver let me on almost immediately though, I got the impression this was not the first time he's had a ticket fail to scan.
It would not have surprised me if either of the McGills of First drivers had refused entry to me based on the ticket, which was valid, based on the machine. Doesn't give me confidence to use it in other areas where I am more dependent on the bus