>How does that hurt me?

Because lit orders get front run. Every sophisticated participant/algo is exceptionally efficient at extracting money from less sophisticated participants.

As someone who trades decent volume but doesn't have a fully institutional grade workflow, I have the fortune of dealing with this...

Simple lit orders (posting an order directly to an exchange) will be taking advantage of by both market makers, by HFTs, and by smarter execution algorithms. The algorithms running the bids and asks will widen spreads. Sell orders will peg to one cent below your ask, and if flows start to reverse, they will pull their liquidity and the slower participants get their liquidity swept through (adverse selection).

The next step up is to use something like a midpoint algorithm or hidden order, but hidden orders will be pinged with one share from the robots and you will get sniffed out and positioned against. If they detect size in a midpoint algorithm, the liquidity in the opposite direction will evaporate, and they will "walk" the dumb midpoint algorithm down, take the liquidity, and then reset the mid back to where it was. The list goes on. It's generally an awful environment for "regular" participants.

Moving on from simple improvements available to the more advanced retail space like midpoint algorithms and VWAP algorithms, you have algo routes that are explicitly designed to take advantage of the "lesser" order types. If they are in a position to get a fair fill, they will rest the order in case they see a situation they can take advantage of, and only take mid fill if the outlook deteriorates (this is all millisecond time frame stuff, but the orders will be worked in an automated fashion throughout the day - time frame is configurable).

On the more developed institutional side, liquidity is sourced in dark venues designed to ward off HFTs and front-running, or sourced in fair flash-auctions which are again designed to ward off hfts and information leakage from the auction spawner.

So the argument would be that perhaps the modern developments like batched flash auctions should just be the new baseline, and designed so that all of the participants feeding into them get an equivalent quality of fill.

These "phenomena" are fairly significant. Let's say you have a 100k position in a smaller cap stock. You may move the stock down a few percent if you start walking down your order and it becomes clear that you are looking to take liquidity. Vs 100k in one of the more advanced order routes where you're basically going to get filled near mid. And of course it goes without saying that 100k won't even move the needle in the institutional routes.

For a while I got so sick of it that if I was looking to buy back my short options (the same things happen in the option space, but with more slippage), I would stuff a basic midpoint algorithm on the underlying, it would be sniffed out and liquidity would evaporate, price would fall, and I'd slam the ask to buy/cover my short calls on the price drop. At least I could get a fair fill when I played two different areas of the market complex against each other... It's just a pain. To the average participant, they will find that liquidity is there when it suits the counterparty, yet not there when they need it.

NBBO/best bid offer itself can be illusory. There are many situations where if you sweep the bid, you will get a fair fill, but I'd you just hit the bid price, you will essentially take off the very small front order of an iceberg order, they will run their calculations, and the liquidity pegs a cent below you if it suits them. That's how it works.

This goes for all areas of the financial market, including the bond market itself, and it contributes to systemic fragility in addition to harvesting retail money.

Granted, almost no retail participant is actually shipping orders directly to exchanges like I laid out. They are going to payment for order flow routes. These are actually fairly efficient, but again, remember that if you are posting a bid or ask, exchanges pay you (yes, you actually net money, albeit small) to post these orders, and anyone feeding into PFOF routes is getting this income taken from them. The frontrunning risk in the payment for order flow routes is also much more severe, since your order is getting blasted out in all directions before it is posted. So when those sorts of routes go wrong for retail traders (ex making the mistake of posting a large order during a major market event), they're could catastrophically get screwed.

It's also worth noting that retail does have access to a relatively Fair auction system though. Open and closing auctions are probably the best ways to fill orders. Just be careful not to ship too much size into them since a large enough net imbalance (say in a small cap stock) in a closing auction will have the same "walk down the price" effect that happens with midpoint orders.

Personally I think that the institutional flash auctions are pretty neat. For my understanding this sort of liquidity sourcing is growing. I would think that this sort of functionality could be regulated and integrated into the base level market venues.