When I was talking to a safe installer years ago, he was telling me about how the mechanical high security dial locks used in government facilities are actually designed to be shielded from radiation.
When I asked him why, he said that, apparently, someone discovered a portable X-RAY machine can make figuring out a mechanical combination pretty easy.
This was one of the driving forces behind a shift to electronic locks. X-RAY an electronic lock and all you can see is the circuit boards.
The good thing about mechanical locks is they've been around for a hundred years, so they have a proven track record.
The bad thing about mechanical locks is they've been around for a hundred years, so they also have well known weaknesses.
I doubt any thief is lugging around x-ray equipment, but there are many other ways getting into the safe. They have auto-dialers on the market that can do a brute force attack in a matter of hours. The smart autodialers can reduce the time it takes from hours to minutes. The actual concept of making one isn't that difficult, they've been made by college students, and I've even read of a simple one made from LEGO's mindstorms.
Now the chances of running into that level of sophistication is pretty low, maybe. If you're unfortunate like this
person , and someone knows what you have, then a few thousand dollars might not seem like much for potential payoff of millions.
However, regarding the switch to electronic, I think one of the main reasons was more for convenience, not so much the added security. It's a lot easier to teach people to punch a keypad than dial a combination lock, it's easier to change combinations, and less maintenance too.
Regardless, I don't think the locks are the weak point of any safe, just as the locks aren't the weakest link in a house. It's a good discussion, but not one that I think people need to really worry too much about.