There is no single rule that is always true. This is one among that. Why I say don't invest in ULIP is because most investors choose ULIPs for wrong reason.

The wrong reasons that the agents use to sell ULIPs are -
1. You can stop your premium payments just after 3 years.
2. You can withdraw most part of your premiums soon after 3 years without any charge and without discontinuing the life cover.
3. You will get better returns in ULIPs than MFs - they may show the returns at various periods (less that 3 years only of course).
(This is because the expenses are taken by cutting down your units not your NAVs, so your NAVs will grow higher whereas your portfolio value would have actually came down.)
4. ULIPs will get more returns without any risk because ULIPs will invest in very large company stocks only (which has declared dividends continuously in the past 6years.)