Personally: - lack of the entire article in the feed, Miniflux and some others try to download it if needed but does not work flawlessly; - lack of RSS/atom s

Personally: - lack of the entire article in the feed, Miniflux and some others try to download it if needed but does not work flawlessly; - lack of RSS/atom support at all for few but not so few websites; - long push against RSS by Big&Powerful that makes too many simply not anymore aware of it's existence, to a point that for many site that still have RSS finding the feed URL from the page source is the sole way to discover it. The rest is far less dangerous IMVHO :-)