This is right: finalizers are difficult to combine with garbage collection for several reasons. As you say, there's no guarantee that the garbage collector will find the finalizable object in a timely manner. Additionally, if it's a conservative collector, then the finalizable object may be pinned by an ambiguous reference and the collector will be unable to free it. In multi-threaded programs the collector runs asynchronously from the point of view of the code, so if the garbage collector calls the finalizer then the finalization code may not be able to depend on invariants of data structures. It is hard to write correct finalization code under these conditions.
Comments
This is right: finalizers are difficult to combine with garbage collection for several reasons. As you say, there's no guarantee that the garbage collector will find the finalizable object in a timely manner. Additionally, if it's a conservative collector, then the finalizable object may be pinned by an ambiguous reference and the collector will be unable to free it. In multi-threaded programs the collector runs asynchronously from the point of view of the code, so if the garbage collector calls the finalizer then the finalization code may not be able to depend on invariants of data structures. It is hard to write correct finalization code under these conditions.
A 2002 technical report by Hans Boehm discusses these difficulties. http://www.hpl.hp.com/techreports/2002/HPL-2002-335.html