For ~20 years now, basic USB and WiFi connection have been in the category of “mostly expected to work” – you do encounter incompatibilities but it tends to be unusual.
Bluetooth, on the other hand, seems to have been “expected to fail or at least be flaky as hell” since Day 1, and it doesn’t seem to have gotten better over time. What makes the Bluetooth stack/protocol so much more apparently-unstable than other protocols?
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Having worked on devices that implement the Bluetooth spec, here’s the real answer: Bluetooth would work flawlessly if everything just implemented the spec.
The problem is: Nothing implements the spec correctly.
If you make wireless ear buds, for instance, and make sure they follow the spec to the letter, everyone will complain about your device because they won’t connect to anything reliably.
So you have to figure out the common part of the spec implemented by most devices, and the common way they violate the spec, and you implement that. And there’s always variance because no device completely agrees on eBay they’re doing, and so there’s a lot of errors in every negotiation, and they differ from device to device.
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