As far as I'm aware Australia and the US have no laws on the books stating compensation. In addition, even compensation friendly places like the EU have carve outs for weather and other items supposedly outside control of the airline.
Based on my spreadsheet tracking various UA compensation offers, what you received is more than fare given the circumstances (what are you expecting UA to do, get rid of the storm clouds?). At the end of the day they got you to the destination safely albeit not on time.
The other question you had was whether UA can issue miles as compensation. Yes, they can and frequently do give the option of taking miles instead of the travel certificate. Based on my spreadsheet I would expect that to come out to 10,000 UA miles. Note : UA can only issue miles for their FFP and not Aeroplan which is entirely managed by AC these days. You can usually do this by filling out the
usual United Cares form or writing in to PremierVoice if you have United Premier status. IMHO the travel certificate is more valuable because you can
book revenue flights using them which will accrue frequent flyer and elite status miles on UA or any *A partner. That being said, the miles may be useful if you don't want to tie yourself to revenue flights on UA.
Safe Travels,
James