Jeremy Horowitz, regarding third-party band makers use of the Apple Watch diagnostic port:

Stop before you spend $250 to order an accessory that might never arrive or work properly. […]

While conspiracy theorists will come up with all sorts of explanations for a hidden port, the two key reasons for the Apple Watch to have it are for diagnostics and performing guaranteed reliable firmware updates.

I would avoid buying any accessories that use the Apple Watch’s diagnostic port. That is, unless you fully understand that it might not work at all or will eventually stop working in the future.