American is unambiguous in English because in the English speaking world there is no continent called America and no one uses the term American to mean they are from the western hemisphere (in English that is).

>Googling around shows I'm certainly not the only person using the term "USian".

You're not, but nearly everyone using it is doing so out of some desire to avoid a non existent controversy. It's also used so rarely that you'll often run into people who've never heard the term.

In addition, it's very difficult to pronounce and will therefore likely never be widely adopted. Do you say "you-es-ians" when speaking?

It's a very similar situation to Australia. There is more than one country that is part of the Australian continent, yet people from the Commonwealth of Australia unambiguously refer to themselves as Australian.

Yeah, in my mind it's you-es-ian. Which doesn't seem particularly awkward or hard to say to me (but maybe a strange until you're used to it).

It just seemed a natural term to me. I promise I wasn't using it to avoid a controversy (which I'm not even aware of!). In my experience with spanish speakers, they'll usually just say "gringos" (not as a derogatory term), but they'll also happily use "americanos" -- and don't seem to have any problem w/ it. (Tho Spanish it is literally an ambiguous word; you have to deduce from context whether it means from the USA or from the continent.)