Why Hang Seng Index or any other China/HK Tech ETF over Stock Picking?

An ETF is a type of security that tracks an index which comprises of many stocks. Hence I would think that it is more diversified across the many sectors, maybe not so on the countries. And volatility might not be that great as compared to a single stock. Probably the con is that there might be some stocks in the ETF which you have no idea of or no interest in.