Is it really a good idea to keep such a vital instrument unprotected? Or is that round metallic thing strong enough to resist impact and fire?
The casing of the ULB is itself designed to be strong enough to withstand the forces expected when an airliner crashes into an ocean, lake or large river (which are the only circumstances where a ULB is useful).
See teardown video starting at around 4:45
Copies of the relevant standards cost $70 each. However, a 1968 report from the early days of ULB development says
As voretaq7 notes in a comment: ULBs need to be external to the FDR/CVR canister because they need to have batteries changed regularly without affecting the integrity of the data recorder canister.