I've worked at finance places where a lot of BL is in the DB. Testing is a trade-off. You can test the end points of stored procedures that the app has access

I've worked at finance places where a lot of BL is in the DB. Testing is a trade-off. You can test the end points of stored procedures that the app has access to for example. Although where I worked admittedly there was no automated testing. You can do CI if you have a way to deploy changes to scripts automatically. The code doesn't need to be a mess but admittedly PLSQL doesn't have the same nice abstraction patterns as OO so it does get a bit W.E.T. sometimes.