Commission free = orders are sent to a predatory POF HFT like Citadel that trades against your order to profit at the expense of the trader, they can make a few dollars to hundreds - depending on the # of shares & bid-ask spread.
Most brokers were already POF, their customer's will save - although they could save a lot more routing orders to the IEX, dark pools, or by using more complex order types.
HFTs back off when volatility spikes, making fills during adverse conditions more problematic.
The noobs will take over trading to a whole new level & the scalpers will get fills when price rolls over - they will be the ones getting scalped. Pros will not use CF orders, it would make the true cost of trading rise many fold. Limit orders are better than market, but keep in mind there is a reason why pro trades abandoned plain limit orders long ago.
It depends on the order types though. I'm curious if auction orders are supported in Lite, they all end up in the same place. It's also interesting whether Lite SMART orders and Pro SMART orders are equal or are there differences. If they are the same, then for most it will be better to switch to Lite.