I just switched from using a blocking link tag to loadCSS today which indeed loads it asynchronously. I'd say 1/5 times I am not seeing FOUT.

However I'm not seeing FOUT on Safari for Mac and iOS and Firefox for Mac at the moment.

*edit: I definitely understand that you're right Paul. If the website loads slow, FOUT is expected. However, I'm seeing it more than what I expected to with loadCSS combined with this bug report I found. So I'm kind of leaning toward the idea that it is a bug in Chrome 50. If not, I'm wrong and it's only my loadCSS causing it.