The first half of the 19th century was the "iron era" of impregnable safes. With enough iron, you could make something that would effectively resist physical attack. Explosives weren't powerful enough yet - dynamite, nitroglycerine, TNT, and C4, let alone shaped charges, were in the future. Black powder would just leave scorch marks. Acetylene torches and thermic lances hadn't been developed yet. High speed steel drill bits didn't exist yet. Cobalt, carbide, tungsten, and diamond tools were a century away. Power tools required a nearby steam engine. Hammer and chisel was about it, and banging on cold iron with cold iron is a very slow process.

Theft from safes thus focused on lock-picking and getting hold of the keys. The latter was usually more effective.[1]

I can't find a picture on line of the setup used to pick Bramah's lock, but I've seen a drawing. The lock design was much like today's round bike locks - push-down pins. There's an easy way to open those today using a Bic pen cap.[2] Modern ones are just pin tumbler locks in a different form factor. Bramah's lock was a lever lock in round form. Hobbs used a custom-made picking setup with a thumbscrew and calibrated dial for each lever. This allowed setting any combination of depths and reproducing it later. Now exhaustive search was possible. Hobbs took most of a month to do the job. It wasn't really a useful attack, but it shook people up.

[1] https://en.wikipedia.org/wiki/Great_Gold_Robbery [2] https://www.youtube.com/watch?v=LahDQ2ZQ3e0