A set screw adjusted block is like turning the handle of a water faucet.  You keep turning it till you get the desired result.



This means everyone has to go through a trial and error process to determine the sweet spots for their own requirements.



Once that is figured out, you have to remember how many revolutions/clicks and what direction to go for your desired result and you need an allen wrench to make those changes.  So make sure you have one with you if you plan on making any changes.



You have no idea what your port size is with a set screw block as it is just pinching the gas off by having a screw occlude the port in the block.  



This restricting point gets hit with all the gas coming from the gas port.



A carbine gas tube is about 7" from the breech and so that is about 17K PSI.




Imagine a blowtorch hitting the tip of a needle, it is going to erode.  See picture below of what an eroded gas port looks like.



The tip of the restricting screw will start to erode at a 45 degree angle just like the barrel does in the picture below. Guess what happens when your rotate the tip of that screw say 1/4, or 1/2 turn...you are now blocking off a lot more gas than expected.




The Govnah design is based on actual port sizes so you know exactly how big your port will be.  Being port based also means that erosion has to take place over the circumference of a circle as well as the thickness of the regulator plate.



We have not worn one out with heavy full auto fire either.  You can read about that here:http://micromoa.com/ak74-test-barrel/.  Also have some videos about that on the site.  Port size validation can easily be verified by removing the regulator plate and sticking the shank of the largest bit into the port...all without removing the gas block.



As discussed on page 4 of this presentation by Crane: http://www.dtic.mil/ndia/2003smallarms/john.ppt, the milspec port size for the MK18 is .070 while DD gasses their commercial barrels to around .081~.082 since they know the end customer may be feeding them Tula or other lower pressure ammo and they don't want to deal with a return.
With the Govnah design, you know what setting the block is in by visually looking at it or in complete darkness by feel.  



You can't do that with a set screw adjusted gas block.
We have SWAT teams using Govnah equipped rifles and everyone on the team knows what setting does what.  If a rifle gets passed to a different officer, they don't have to test fire it to figure out what setting the gas is in before entering a building.  Just look at it or feel it instantly know.  
Being tool-less you can adjust the regulator plate with the tip of a bullet or other object to push the regulator plate and in some rail systems you can do it with your finger.



We recommend having the orientation of optimal-unsuppressed/suppressed/adverse and in the case of the DDMK18 for a milspec configuration that would be .070/.049/.82 (left/center/right).  So say you are running with a can and regulator in the middle position, need more gas since your gun is too fouled?  You don't have to think which direction do I go and look for an allen wrench.  You push left or right and you will get more gas.
Regarding the MK18, here is a thread discussing that configuration with the Govnah: http://www.ar15.com/forums/t_2_543/258195_MK18_and_Govnah_specs.html
So decide for yourself if those advantages are important to you or not.  Good luck with your build.