This argument can go 'round and 'round for days, so I will try to take some of the steam out of it with this explanation, not saying I will be successful... LOL
The Direct Impingement system is the way the platform was designed. It harnesses gas from the round and directly cycles the bolt/action.
When the M-16 went to Vietnam, there were complaints that it would not work well if dirty/muddy.
Changes were made, education was initiated and the weapon system moved forward.
At the same time Good Ol Mikhail over in Mother Russia had developed the AK platform, which was a knock off/copy/ hommage to a German design.
Ths system is a piston, or op rod, type platform. The gas is harnessed near the bore and it drives a piston to cycle the action. As there is less overall travel of gas, the "piston" or "op rod" covers the distance with solid metal and does not allow the same kind of dirt/mud/grime.
Wars come and go, AK's come to the states, people start getting interested and then we have the AK vs. AR battles/tests/comparisons.
Someone got the idea of why not combining what is great about the AK, the fact that it will run no matter where you are deployed, with the more familiar AR platform.
There was money to be made - need I say more?
From a very novice gunsmith wannabe standpoint, you get this:
The AK is not as accurate as the AR - regardless of who made each model. If you compare skill level of maker to skill level of maker, the AR is more accurate. The piston system of the AK causes flex in the barrel with each follow up shot, so accuracy is compromised past shot one.
Adding a "piston" to an AR does the same thing. It is "more reliable" but you are losing accuracy because of the way the cycling of the action must work, you have harmonics along the barrel.
At street to street, house to house type of fights will this affect the "average" shooter?
NO! Absolutely not. Out to 50 yards, Minute of Bad Guy is going to be Minute of Bad Guy unless you are trying for a head shot.
At 200 or 300 yards?
Yes! A hit with a DI AR might not be a hit with a piston AR. Not that you can't get man sized hits, you can, but the DI is going to be more accurate.
So, you want to build one. The Piston versions are more expensive currently so if cost is a factor, and I guage that it is by your post, it is going to be cheaper to build a DI edition.
I live in the Pacific Northwest. We get rain, wind and usually blowing pine needles and "junk" about 8 months out of the year. Now, I have never gone on a three day creep with my AR, or put a lanyard on it and pulled it through a swamp, followed by a sandbox, followed by a wet concrete pour, but I have never had weather conditions cause my weapon to stop working.
Could it happen? Yes. If you do not take care of your equipment, any piece of equipment can fail, for the most part.
For most folks, the benefit of the AK has always been it will run dirty, cold, wet anywhere on the planet and keep heads down with that distinctive chatter.
In my mind the benefit of the AK has always been the heavier round. Why take the negative, the piston/op rod, and keep the smaller, less powerful AR cartridge?
I feel the Piston AR's are a solution where the problem was created by people Rube Goldberging something to death.
Good luck and feel free to PM if you have further questions on a build.