It's kinda hard not to start everything on fire in the house if the safe is bolted down (which most should be). If you were a real perfectionist, you could measure out the thermite to melt only the exact thickness of the safe. (which I think you are talking about, james) The problem is all the metal of the safe will still get hot and may burn the safe's contents.
I was watching Impossible Heists (episode 5) and the expert safe cracker was cutting through a steel bulkhead door with a gas powered concrete saw similar to
this one (with an abrasive blade). While he was using it he said "Wow, this thing would be good at cracking safes!"
"Lockpicking is what robbing is all about!" says Jim King.
