The garbage collector in 5.4 implements a generational mode. If an object survives the minor collections it may take a very, very long time before its __gc metamethod gets called after is becomes inaccessible, especially if your program mostly creates short lived objects. This makes __gc less useful as a poor man's RAII replacement.
The new "toclose" feature is much more useful to release resources and unlock locks in a timely matter.
(If you use finalizers in such a way that the point at which it is run matters much your code is very likely buggy for other reasons as well)
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