I worked with both DB2 & UDB during my internship 2 decades ago, and while largely compatible with core SQL functionality, that changed quickly after you got off the "normal" path. UDB was released at a much faster cadence, and if you developed to the latest features of UDB, you'd often find yourself unable to deploy against DB2.

A few of my last projects as an intern: 1) storing BLOB objects in DB2 from a .Net app and 2) porting an "interactive" batch-processed COBOL app to ASP.Net form 3) getting DB2 Connect clustering on windows working (I never did get this working despite spending an entire summer on the phone with IBM support).

I've some fond memories of working with the mainframe. Like the time I managed to crash the entire development LPAR with a specific SQL query that broke the DB2 query optimizer... The phone call from the NOC was immediate and went something along the lines of "I don't know what you did, but don't do it again, you just took down all of development". Development was something like +10k users...