Polyfills won't break your page and random, worse case you get worse performance over native. However the point of a polyfill is to do exactly what the native method would have done if it existed
It will break your page if you aren't sent a polyfill when you should have been. Imagine running a browser without Object.observe, spoofing an agent of one that does have it, and you aren't sent it. Suddenly data-binding doesn't work at all.
Yes but the way mozilla has chosen to implement this is to send the code over the wire based on the clients UA. So if a method doesn't exist that should exist the client will never receive it.
It would be cool if they allowed you to disable this feature, and just use feature detection and always load the code for the requested item.
The polyfill service was created and is hosted by the Financial Times. Mozilla is not affiliated.
There are good reasons why you cannot make a good choice of polyfill prior to knowing the browser family and version, primarily due to polyfill variants (more details in the Hacks post).