based on my own experiences with a number of cases, I believe it depends on two factors, one of which you unfortunately have NO control over. The first is the way you word your application so it is very clear that you meet the impairment criteria, or your combination of ailments is such that you "equal" the impairment criteria.
The second, and one out of our control, is the level of intelligence of the person reviewing your application. I've honestly had several clear cut, meet the criteria with out a doubt applications and they get denied. Then I've had some where I've told my client, "this is going to be a tough one, an uphill battle, but I'm confident you WILL get benefits", and they get approved immediately.
But wording the application correctly, and being very familiar with the impairment listings is the MOST important thing you can do.
Yes, SSDI is based off your earnings history, it has nothing to do with geographical location