Are the completely safe and is there a limit to how much you can store?

They're completely safe. I've seen no limit. Pretty useful if you want to move stuff between house locations.

Ok cool thanks. I haven't been to vegas yet but I'm sure they have one?

Short answer, yes.

In FO3 I had loot disappearing from a mailbox on me. First I thought it was raider scripting (to loot mailboxes), then I thought it was cell overloading (where things are overwritten FIFO)

finally I figured it out.








I was looking in the wrong mailbox :facepalm:

So, other than pulling an Andyism, they should be safe.