That doesn't tell me very much. Should I forget about the Valley then, because visas are too hard to get? I've got better things to do than dealing with immigration issues - it seems a big drawback.
The next best option is probably London. I'm guessing it's the best place for start-ups in Europe?
Ease of immigration/visa to US depends on which country you are from. So ask around your place. If anyone has immigrated to the US they are the ones who will be capable of giving you the best picture.
And yes, if its too grim, forget about the Vally. Yes, its sad and disappointing especially with all the pundits stressing the value of location. But be wild. Its a constraint, and last time I checked constraints promoted creativity. So use your resourcefulness to make up for your lack of resources. Its fun. Really!