This study [1] does a good job rigorously comparing 1/N to various alternative trading strategies. It also talks about situations where 1/N is more or less likely to dominate.

"Our simulations show that optimal diversification policies will dominate the 1/N rule only for very high levels of idiosyncratic volatility."

[1] http://faculty.london.edu/avmiguel/DeMiguel-Garlappi-Uppal-R...