Market orders are well-suited for securities that are traded in very high volumes such as large-cap stocks, futures or ETFs. For example, market orders for the E-mini S&P or a stock such as Microsoft tend to fill very rapidly without issue.
It’s a different story for stocks with low floats and/or very little average daily volume. Because these stocks are thinly traded, the bid-ask spreads tend to be wide. As a result, market orders sometimes get filled slowly for these securities, and often at unexpected prices that lead to meaningful trading costs.