Genuine question, but how is functionally more limited? You can still run any executable, munge text outputs, and... Well, isn't that what just about any shell do?
On top of that, you can play with objects on the pipeline, which fewer shells support.
Edit: perhaps you mean in comparison to scripting tools, e.g. Python? I had read the OP to be asking more about it as a shell, I think.
Just as I was saying to myself "I'm pretty happy with the Garmin I have, I don't need a new Pebble," here come Pebble's marketing department with a reason!
... What do you mean this is an announcement from Garmin?