> I once printed out the BT5 spec (just the core), for Ss and Gs (on my employer's dime).

I've never dealt with Bluetooth myself, but once heard/read that a company could follow that spec perfectly and their device wouldn't work with large swaths of devices. The claim was that companies implement something that's Bluetooth adjacent but there are some non-conformances that everybody "just does" to make it work the majority of the time.

I'm curious if your (and others) experience align with this?

Well, the equipment that my company made was really quite primitive, BT-wise. They also only supported Classic (BR/EDR), and only then, for control stuff (shutter release, aperture adjust, etc.). Image data was sent over WiFi, and BT was used to help set up said WiFi.

Apple is probably the biggest offender, when it comes to custom implementations, but BT explicitly allows vendors to do custom implementations, so they aren't actually breaking any rules.

The BT spec defines 5 overly complicated ways to do any given thing but most devices only support 1 or 2. Multiply by the number of things that need to be done. In practice the compatibility situation is "test all pairs" and just as bad as if there were no standard at all.