I have a bit of experience with a commercial one and updates were worse then death. They would lock everyone out of the system. And you couldn't run multiple customer on a single system since you could only update one database at a time.
The canned query system (visual, based on venn diagrams) was hot shit. And when drilling though TBs of data, joining and eliminating, it was lightning fast.
But when we went to code against the actual engine, all hell broke loose. Then the MGR decided to toss it offshore (hehe haha) and that took it to it's death, getting the mgr and his boss canned in the process.
Not often I see poor tech devisions that I argue against get punished so nicely.