One problem with feeds is many sites don't have feeds and people blame RSS rather than the site not implementing it. Another problem is noise: most sites just offer a single all-or-nothing feed, while they categorize/tag posts on their site. If they offered a comprehensive set of microfeeds that would help adoption I think (or include the category metadata). Another problem is truncated feeds, requiring users to click links and navigate to the original site to read each item. While some premium readers claim to offer complete news items, this is probably against the wishes of the site and may be circumvented similar to the add-blocker arms-race. In my opinion if a site model requires users to visit it, then they shouldn't offer feeds at all.

Lastly some feeds are behind logins (e.g. forums), not all readers support these feeds.