You expose yourself to currency risk as well with that strategy. I primarily worry about single company failures and wobbles; if the US economy implodes, the rest of the world will be caught up in it.

But you're right, you should balance with non-US as well. I've steered clear of them, myself, but that's not per se ideal practice and I'm overdue for a rebalancing.

But, to be clear, I was sketching an entry level into investing, not a more sophisticated approach.