Assuming that you can use a familiar as a safety-deposit box (because the DM's has ruled so), how would this interact with an magic item that makes an extra-dimensional space?
What happens if you give your familiar a Bag of Holding, Portable Hole, or similar item, and then you dismiss your familiar to its pocket dimension? In particular, such items when put in each other tend to burst and cause problems. Does that happen in this case as well? If not, why not?