and that's why sector specific indexes are not "good" - only broad market (heck, even global) indexes are worth passive investing for.
A nasdaq index is no different from any other thematic index (like an oil index, or a robotics index). Thematic indexes tend to fail the investor in the long term for capturing beta. But because of lack of knowledge of the _actual_ academic research by retail investors, a lot of clever marketeers sell the idea of a thematic index as tho it is similar to a broad market index ("safety" and diversification).
A nasdaq index is no different from any other thematic index (like an oil index, or a robotics index). Thematic indexes tend to fail the investor in the long term for capturing beta. But because of lack of knowledge of the _actual_ academic research by retail investors, a lot of clever marketeers sell the idea of a thematic index as tho it is similar to a broad market index ("safety" and diversification).
Caveat emptor.