I've just pushed fixes for properly respecting private category permissions.
I've also removed references to the Set object, which should fix the older android devices. I'd also be cautiously optimistic about it fixing IE11, but I haven't been able to get my hands on a testing device so can't confirm that yet.
Haven't been able to repro the issue @Sylvain7 is describing; does it happen on all browsers? Does it happen on discourse-babble.com? Any errors in the console?
I've also made some headway on the ios experience, which I'd characterize as 'pretty useable' at the moment, although it does occasionally scroll you to an odd place after the keyboard shows up. Will continue to chip away at this one.