Ten years ago, I added an Atom feed to this website. Before that, there was an RSS feed—version 1—and for a while I had an RSS 2 feed. But in more recent years I disabled RSS and have used the Atom feed exclusively.
The reality for Atom is that, not only is it difficult to write valid XML by hand, but people also have trouble writing programs to generate valid XML. Once a feed is available, it needs to be parsed. Even parsing valid Atom feeds is no simple task, but the implementation challenges mean that there are also a bunch of broken Atom feeds in the wild that feed reader writers have to deal with.
Enter JSON Feed, written by two individuals with a lot of collective feed-processing experience, with the stated goal of being “simpler to read and write” and “less prone to bugs.” It has been adopted widely already, both by publishers and feed readers.