Here's what I use my 8 slots with:
- Compass (hopefully to be replaced by a map program at some point..): no JS side
- Multi Timer : no JS side
- Twebble: working flawlessly except posting tweets, which doesn't seem to work. Not that I would never like to do that either way since keyboard is just painful. Also viewing my own profile is broken but it seems broken under Android too. In fact, I can't post tweets from Android either...
- Weather: flawlessly
- Authenticator: didn't try the JS side very much, but it seems like it's just configuration UI, and the actual logic is done on the watch. Good.
- Misfit: no JS side, but I don't understand where this application gets its data from. It measures "deep sleep" even when the Pebble is turned off.
- Yelp: working flawlessly. I had to patch the JS side a bit because the original was insane regarding timeouts (e.g. "get me a fresh GPS fix in 2 seconds, stat! and if you try to be sneaky and send me a cached GPS position that is just 1 femtosecond old I WILL notice it and DIE HORRIBLY!!"). I suppose Android plainly ignores those timeouts.
- Evernote: the JS side takes a whopping 10 seconds to load on my Jolla (taxing QJSEngine, I guess), creating a very noticeable delay. I sometimes get "undefined" when trying to browse my notebooks, however no JavaScript exceptions actually fire. The word "undefined" comes from the Evernote servers.... and in fact I get the same behavior on Android too, just less frequently. Those guys ship a XML parser written in JS, so...

Note I haven't tried any actual "fully javascript" app (e.g. cloudpebble) yet, so if you've been able to run one, I would be interested in hearing reports.

By the way the next firmware upgrade brings multi-language support to the table. However changing languages on the watch seems virtually the same as uploading a new firmware.
I'm not sure how "safe" is Pebble's recovery mode, so will probably not experiment with firmware loading.