I am a native English speaker and would say unburnable, but I'm from Wisconsin and we tend to put un- on lots of things. My coworkers give me grief because I "unpeel my bananas" . Google Manitowoc Minute Unthaw for another example.
Why is this not acceptable? It portrays the same understanding! I noticed with Swahili one has to literally state verbatim what the language translation has been programmed as. Duolingo Team, please look into this