I saw a post here few days ago talking about the "battery widget" not appearing in Notification Centre.
As I said, it really depends on whether the maker of the Bluetooth device adds the support for the battery info or not.
For my case, it supports it as shown in the widget and the small status bar icon on the upper right. And this small Bluetooth speaker is just Singapore dollars $9 (think about USD $7).
So it not only official Apple accessories like the Apple Watch supports it.