I love it!
I'd stop monitoring extract failures since Tableau Server has been removing the schedule after 5 failures...however it seems I have a lot a tabcmd users who keep trying to refresh failing extracts
I'll go after them on Monday!!
Did you have any success with integrating the Db Class/Type into the workbook? It would be nice to see what kind of data sources the extracts are running against and to see if a certain database is having more errors more than others. I haven't been able to get the database class table to work properly though.
Workbooks are treated as a single extract refresh operation, when it comes to the background_jobs table. So while you could join it (using some arcane magic) to the Workbooks table, then to Data sources and then Data Connections, you'll end up with duplicate rows for any workbook with multiple data sources in it. Which may make the analysis a bit more difficult.
I've built some curated data sources that you can blend together to get what you're after--or pick apart the connections to modify as you see fit. Check 'em out here.