Every agent you ask will have a different number for their success rate.
I feel very lucky - and all my short sales (where I was the listing agent) have successfully closed so far. I had 2 where I represented the buyer (same buyer) - one time the listing agent and the negotiator/attorney didn't know what they were doing, the other time the buyer had a problem with money, at the last minute. I currently have one listing where the bank is Ocwen - they seem to be the absolute worst.
I hear great things about Chase being pretty easy (if you have a true hardship and defaulted on your mortgage). Success rate does not depend on where the property is located.
You'll have two options - HAFA and a regular short sale. Both work fine, and there are specific qualifications for HAFA short sales. Check out this site: https://www.chase.com/chf/mortgage/hrm_hafa
To improve your success rate, I suggest to work with a CDPE designated agent experienced in short sales. You can find one on http://www.CDPE.com
website (Certified Distressed Property Expert).
You need someone very knowledgable on your site.
Overall, different people quote different success rate. Not everyone is up to speed on short sales, so if you work that into the equation - the success rates are pretty good.
Best of luck and hope this helps,
Beachfront Realty, Inc.