Someone moving from (say) Cambodia, however, will face an entirely different situation. They plainly have any number of motives to get the hell out of Cambodia and into a big rich country. These motives can and do lead to fake marriages, fake information, everything. The application will be given a far greater level of scrutiny and will take a lot longer.
Hell, don't take my word for it, Australian immigration spells it out. Here's the official "assessment level" (ie, assumed risk) of people applying for student visas here:
http://www.immi.gov.au/students/students/chooser/574-nonjs.h...
Plan on a miserable experience if you're not from a country in level 1. And you can be sure there's a much more detailed one for internal use, for spouses, etc.
I'm originally from an African country, and trying to get into the US was next to impossible despite my qualifications (valid), experience (in several Western countries) and financial position (good).
Five years on, with New Zealand citizenship, I can travel visa-free virtually everywhere, work in Australia without paperwork, and can expect a short application process to get into the US, based on the experience of a friend of mine, who had the same progression.
It sucks if you're on the other end, but to some extent, a greater degree of scrutiny is warranted when your origin is a country you have motive to get out from and to commit fraud to do so.