Many registered reps selling 401(k) plans in the small to mid-sized market would have you believe this.
To be clear, I have enormous respect and admiration for American Funds as a fund family. They offer a number of excellent funds. They have a deep management/research group. I use several of their funds in 401(k) plan line-ups and in the accounts of some of my individual clients (no-load share classes).
Contrary to what these registered reps may tell you, an all American Funds lineup is not, in my opinion, a complete 401(k) solution.