The small-print of the "free" brokers can also be quite, erm, "interesting".
See, for example, the following gem from Robinhood :
I further understand that when I send a market buy order through Robinhood Financial’s trading system, the trading system generates a limit order up to 5% above the last trade price, and then Robinhood Financial sends the order to an executing broker.
Now, to be fair to Robinhood, they do appear to allow you to explicitly create limit orders. But its hard not to think about the fact that the sort of people attracted to "free" brokers, will also be the sort of people who have no clue about the basic differences about a market and limit order.