Below is taken from CFAI text
A manager universe—or manager peer group—is a broad group of managers with similar investment disciplines. Manager universe benchmarks allow investors to make comparisons with the performance of other managers. Managers are typically expected to beat the median manager return, which refers to the manager return that splits the sample of managers’ returns in half. Manager universes are typically formed by asset class and the investment approach within that class. Hedge funds are often evaluated relative to manager universe benchmarks , provided by such vendors as Credit Suisse/Tremont, Hedge Fund Research, and Lipper/TASS. Private equity funds are also commonly evaluated in this way; examples of data vendors are Burgiss, Cambridge Associates, Preqin, and Thomson Venture Economics. Investment magazines are prolific providers of peer group comparisons for investment funds (e.g., mutual funds in the United States and unit trusts in the United Kingdom); thus, a mutual fund investing in global equities might be ranked among all mutual funds with similar objectives over various time periods.
I have come across questions where it says manager universe is not a valid benchmar. What am I missing?