I honestly think you're better off with a safety deposit box and some acid free paper. If someone has unfettered access to that room there are far more valuable things than a bitcoin wallet.
Depends if you that the bank staff not to look through your possessions while your gone. While jewellery cannot go missing from a bank safe deposit without some big questions, if some scrupulous staff member copies down your private key/mnemonic they could move the coins without having to steal the actual paper. And with the paper still where it should be you have no way to prove that it was the banks incompetency.
Not likely to happen, they keep very good records of who accessed what box when.
And there is no reason to tell them what is in your box, so the only possible danger is an employee than randomly looks, and that's the kind of employee that gets caught and fired.