The thing you should be concerned about is understanding all the risks with a particular asset allocation. Then, understanding yourself enough to know how you would respond to different scenarios that could happen in the future. What if the US has a 20+ year bear market like Japan is currently experiencing, or like what happened in the US during the 30s-40s and late 60s to early 80s? How then would your 100% US equity allocation look? What if you decide to go 50/50 stocks and bonds and the bull market continues for 10+ years? How will you respond?
To me, you have to become a good risk manager FIRST, before you can become a good investor. It's ok to be 100% invested in US stocks as long as you understand the risk you are taking, and are ok with it. If you aren't, then it may be time to reconsider your asset allocation.