The downside of setting a limit order is that you only get executed when the market moves against you, OR just don't get filled.

For retail order sizes and holding periods, that's probably fine as long as you place orders close to the current price.

There is also the case when no one else wants to sell (buy) and bids (asks) way out number asks (bids), especially on contracts that people are using to cover for their other trades. There's plenty of times where I massaged the price up (or down) to where I wanted it more, and dumped/slurped up contracts (at least confirming by checking the bar once it filled and seeing that the price I got filled at was the highest/lowest price of that bar). Granted, it doesn't happen every time, nor do I count on it.