StackOverflow has some good comparisons of Microdata, RDFa, and JSON-LD, especially in answers by "unor" in 2014, 2015, 2015, and 2016.
Google specifically states "Google recommends using JSON-LD for structured data whenever possible." But when I look at the page source of a Google search, the very first content of the page is:
<!doctype html>
<html itemscope="" itemtype="http://schema.org/SearchResultsPage" lang="en">
The itemscope and itemtype attributes in the <html> tag are microdata. So why is Google using microdata when it tells us to use JSON-LD? Is there something about the use of structured data in that tag that makes it unsuitable for JSON-LD?