1. You note that the browser still powers the apps even though they are 'native'. What sort of performance limitations does this bring up?
Well, it runs with the same performance as it would in the browser. So it depends on the quality of the phone's javascript engine and HTML5 support. Mobiles are a bit behind desktop browsers so it might be a while before most games run really well this way. For example most mobile browsers don't yet have very fast javascript engines - but they're all moving towards fast compiled engines like desktop browsers, and they're all making moves towards hardware accelerating their canvases as well, so it should only be a matter of time until they're fast enough for most games.
. Will the service cost money in the future? as in monthly payments?
Possibly - it's up to PhoneGap, and I'm not sure they're certain of their plans yet, but there's a chance they'll keep a free version. Best try it out now while it's free!
. You also note that only one sound will work at a time. Is this a limitation that will never be fixed or a simple missing feature?
As the r66 changelog notes, it's a hardware limitation. We might be able to work around it with uncompressed WAVs but we'd have to be careful to do that in a way that means nobody can accidentally upload gigantic WAVs to their server that ruins the game download time.