There are non-natives who write well, but it's more of an exception than a rule. So obviously to just cut out the higher chance of low quality, I always suggest to stick to US/UK/Canada. If 90% of non-natives aren't good writers, then it just makes logical sense to cut them out of the game to save the hassle.
The issue with non-natives is that they don't know the nuances of the language. It's those nuances that connect with readers. They seem to write mostly in cliches and focus on specific words. For instance, I was an editor for a company who wasn't paying their writers enough, so all I could hire were non-natives. I had to tell a guy to quit explaining everything as "ubiquitous." Using it over and over makes for poor reading. I had to tell another to stop explaining everything as "simple." Oh, programming complex algorithms is "simple." That was his adjective for everything. It's redundant but they don't have the vocabulary or understanding of the language to do differently.
Another issue is that they don't understand compelling writing vs writing that just make a sentence wrong. For instance, "Everyone uses Twitter for social signals." Everyone? -- can't write like that. "Most" or "Several" work here, but not "everyone."
It's these problems that lower the quality of your writing. People who are experts in the field pick up on this and move on to another site.