Well, this is used now by about 4000 clients. So you be the judge. There is so many indexes, and spaghetti, just to make sure every possible "relationship" is accounted for. Amazing it works at all.
"Badly". At least once a week someone managed to overwrite a change another client had just made. When I left, they were running a cron to copy it every minute as a "quick restore" when it happened.
Yeah, first real job had the customer database as a VB frontend onto an Access file shared over {whatever Win95 was using in 1996-97}. After a few snafus, the CS / Sales people learned that they had to single-task with the database.
Just using an XML file is pretty crazy though.