According to the comment in the config file for FastConnectable, "the tradeoff is increased power consumptions". Doesn't say anything for JustWorksRepairing, although in a quick search I read some stuff about security issues, so perhaps that's why?

I've used Bluetooth for a number of devices for about ten years, and I didn't even know there's a config file until now! Thus far, it's been mostly a "just works" experience except for the battery level on by BOOM 3 speaker, but that's their fault for doing some proprietary bollocks.