Not only that, but I have been working with some queries that contradrict this. We had some UDFs that are hyper-simple fetchers of config values from a KVP config table.

I tried optimizing them out. Nothing. Looked at the query plans - 0%. SQL server had reasonably noticed what they were and done a bang-up job handling them.

That's the infuriating thing about SQL server "advice" - it's such a black box that so many of the rules are cargo-cult stuff.