For me, it was the all-in-one built-in solution. I didn’t need an external tool (like mbsync or whatever) to pull my IMAP email from a server just to read it with Emacs. Gnus is able to pull IMAP (and POP3) email messages from a server directly. Sending email also didn’t require additional software. I put a generic example here in case that helps.
I don’t use it as a replacement for elfeed, but then, I don’t read many RSS feeds. I did try to setup RSS in Gnus, it worked… sorta… I kept getting duplicate messages. There is some debugging you might have to do in order to figure out the keys needed to configure your RSS feed so Gnus understands how to not pull down messages you already have read. There is a wiki page on it somewhere, I’d have to go find it, let me know if you need me to do that.