When dealing with userdatas in particular, it can be useful to reset the
metatable in the finalizer to nil or to a dummy metatable, because there
is a specific case where the object is _NOT_ unreachable after the
finalizer returns [...]
I just figured that out, so it does make sense to remove the metatable if your finalizer leaves your object in an unsafe state otherwise. I typically have an explicit method that releases the resources used by a userdata object (object:close() or similar), so I don't particularly care about accesses via an ephemeron table.