This is somewhat subjective, but ala, it boils down to the following:
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Navigation rules in XML are a maintenance hell.
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Using navigation rules suggests that the web application in question suffers from the “one URL behind” problem which causes bad user experience (pages are not bookmarkable).
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Using navigation rules suggests that the web application in question is using POST for page-to-page navigation which causes not only bad user experience (pages are not bookmarkable), but also bad SEO (bots don’t index POST, hence POST-navigated pages are unreachable for public search engines).