> One thing I can fault the Bluetooth SIG is that the spec is very complex and full of optional bits which could lead to many pitfalls for the uninitiated engineers, though Bluetooth chips from reputable manufacturers who know what they're doing, like CSR, TI, SiLabs and Nordic are nearly bulletproof in this regard.

Earlier spec versions aren't just incredibly complex - they're contradictory.

That's what led to so many manufacturers doing their own thing in very slightly incompatible ways. You can pick up two certified bluetooth devices and find them to have different behaviour in the exact same circumstances. In fact, some of the best devices are ones that aren't certified, because they ignore the spec and act more reasonable than it demands.

Apple famously skips several of the mandatory verification handshakes to reduce latency in their AirPods. Not optional, mandatory. Because the spec doesn't work that well in the real world.