It is a giant [bleep]-pile of a protocol. It was only intended to connect one phone to one wireless headset with crappy speech only audio quality and they've since erected a giant tottering artifice on this tiny little foundation.
It won't ever work reliably in general, and it can't ever work reliably in general.
Sometimes you find two devices where the people who wrote the BT stacks and did the hardware were in sync with each other and it does mostly work - the Wii controllers were BT but Nintendo had total control over both ends so they were very reliable. The PS3 DualShock controllers would drop out semi-regularly, but the PS4 DS4 controllers seem pretty solid.
Two random devices from different manufacturers? Total pot luck. Sometimes you get lucky and it's pretty good. Sometimes it doesn't work at all. Usually it works mostly.