I only tested with my smartphone, as it's the only bluetooth device I reliably carry with me.

I'm sure there are configurations that work - blueproximity probably worked well for its author, or they wouldn't have released it! And I gather Windows offers "Dynamic lock" which locks the screen based on bluetooth.

However, having experienced bluetooth unreliability with Linux, Windows, Android and iPhone; and with mice, GPS receivers, cars, access control systems and sports watches; I am confident the unreliability was not unique to a single bluetooth device.