Not only standard itself, but also low quality implementations both in hardware and software. And while major OSes’ BT stacks continue to gradually improve over time they won’t help you if you Bluetooth hardware or device you are trying to connect to (again both hardware and software) are trash. It’s a curse of every open standard, no matter how good or bad it is by itself - there always will be shitty implementations. And if there are a lot of them (like in case of BT) then majority of them will be shitty.