Data attributes are explicitly specified to only be for the site itself, not for external reuse:
Custom data attributes are intended to store custom data private to the page or application, for which there are no more appropriate attributes or elements.
These attributes are not intended for use by software that is independent of the site that uses the attributes.
They are also meant for non-visible data. Using microformats means you can use the text shown on the page as data, rather than a parallel version that may go out of date. This DRY issue is the problem with JSON-LD too, as well as its complexity. I wrote about this with examples.
Microformats are in active use by the indieweb protocols including webmention and micropub. I've written about possible collisions with u- prefixes before too.
Microformats 2 are explictly designed to minimise clutter, and are terser than alternatives that require extra versions of the text like JSON-LD and OGP.
