The only cases I have heard that Google have withheld monies or forfeited them from payment, is when the user has broken the TOS, knowingly or not. There is a lot to know about running adsense on your site nowadays... many things that people just don't understand, and give Google valid reason to decline payment as part of the contract entered into. Comply with the TOS and there should be no issue.
You can only run adsense on specific type sites, even having the ad in your header and if you have a menu that drops over it is breech of the adsense agreement. Know the policy, comply with the policy and there should be zero issue.
I got burned by adsense. I had about 150 GBP saved up on there and one day I noticed a massive spike in the impression rate. The next day I logged in to find they had suspended my account due to 'illegal impressions', so I made an appeal and after 4 months later - still no reply. Because I live in Japan the adsense team in japan were dealing with the complaint. So I posted on the google boards and they said just wait. 8 months later I get an email from them telling me that my appeal has been denied due to illegal impressions from the UK, but when I submitted my claim it states that 'your IP address has been collected', so they knew that I live the other side of the world and still banned ME. So now I cannot use adsense on any site that I ever own. Also I found that my search results dropped in google too, so I have lost out in hits for something I never did.
I won't support google adsense again - plus all they did was give me bloody adverts for handbags. Oh, and the worst thing is they keep sending me adsense emails that I cannot turn off because they have blocked my account.
I moved over to amazon affiliates and that seems to work well sometimes.