is this the argument against cementing in fence posts?

rebuilding a 30 year old fence.
what are the pros and cons of the various ways of setting the new fence posts?
the options I've seen seem to be:
1. setting them on gravel in a deep post hole and packing them with well tamped "gravel base" (gravel and gravel dust)
2. using metal post holders
3. encasing them in cement.

What's the best practice here?

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