Defiance makes a nice action. The best part about Defiance, is all the custom options available, if there are certain features you want incorporated into your action. For the most part, if you want it, they can build it. No other custom action manufacturer that I know of has as many options as Defiance. As others have mentioned, they are generally built "tighter" then other actions - the tight tolerances can certainly cause issues with debris in the field, and applying a coating that has any thickness will exasperate the issues. My first PRS rifle was a GAP rifle with a Defiance action - in really windy and dusty conditions I would have to wipe the bolt down after every couple of stages just in order to keep it running. It got so bad that I could barely keep it lubed, and the lugs ended up galling. I've since gone to a Mausingfield for my PRS rifle, and that action handles adverse conditions exceptionally better. If you do end up with a Defiance, I would recommend a surface treatment like nitride. Avoid Cerakote. If you shoot mostly from a bench or "square range", and if you're not planning on using the rifle for extended periods in adverse conditions, then it's really not that big of a deal.
Here's a video that show's a GAP rifle with Defiance action getting pretty gummed up in the field. Too much dust/debris, especially with Cerakote, commonly causes the issues exhibited in the below video:
Another potential issue with Defiance is that the lug abutments are smaller then other actions. Not a big deal with SA cartridges, but could potentially present setback issues with heavy use magnum cartridges such as a .338LM. My .300NM is built with a DLC'ed and polished Defiance action. It's incredibly slick, and I've really enjoyed the action for this purpose. It doesn't see adverse conditions for extended periods like my PRS rifle, so it's ability to handle debris hasn't really been tested, and for it's purpose it's not so much of a concern. If you are going to use the rifle with LA high pressure cartridges (say 33 or 37 XC, or even .338LM or similar) and/or use in the field for extended periods at a time, the Surgeon actions would probably be a better candidate.
Impacts are built to be pretty resilient in the field. They eat dust and spit it out. They come nitrided, which is an excellent surface treatment option. The trigger hanger is a nice feature as it allows you to "time" the trigger to the action, creating a nicer interface between trigger and action which results in the action "feeling" nicer. Just keep in mind that some stocks/chassis may need some slight modifications to allow for the trigger hanger to fit, though more and more stock and chassis manufacturers are building their equipment to accommodate this feature. They are built to such exacting tolerances, that pre-fit shouldered barrels are being built for them - this means that you can order a barrel to your door and install yourself. No need to send off to a gunsmith. However, the new Defiance Ruckus (and I believe Tenacity) allows for this as well. The Defiance doesn't, because it has so many different custom options available that can change headspace. However, if you send your action off to a gunsmith once, they can save your specs on file and use those in the future to spin up barrels without having your action in hand. There are no custom options available for the Impact action - they come as they are. But they are one of the nicer renditions of the R700 2 lug push feed actions, and I'm not really sure what you would want to change on them anyways.
As far as extractors go, the mini M16 extractor is commonly considered one of the better and more robust/reliable extractors available for push feed R700 type actions. I don't know anyone that would consider the Sako extractor better then the mini M16 extractor, but if that's your preference, then Defiance would be better able to accommodate that.