I recently bought a bangle.js and I'm really impressed. On the first day I had a JS REPL running with their hello world!
The firmware is FOSS but you write JS apps on top. It's really hard to break. There's already an 'app store' too.
The Nintendo Switch has a clever way of preventing firmware downgrades: upgrading the firmware blows an internal fuse!
https://news.ycombinator.com/item?id=27202766
This means that the hardware knows a newer firmware exists.