Product Snippets with JSON-LD + Schema.org - Are there any live examples in Google index?
Update 2016: The documentation got updated and it no longer contains the note. So now JSON-LD seems to be supported for the Products Rich Snippet.
Google’s documentation for the Products Rich Snippet says for the single product page:
We are in the process of implementing JSON-LD support for this Rich Snippet type. At the current time, we recommend using microdata or RDFa.
So this might explain why it wouldn’t always work, currently.