The thing with AKs is they are not all by the "specs" or they was, but got little but more wanky while they were remade for civilian market - this specialy, in europe it must be such a adjustment that prevents automatic fire and prevent remading by common resources to allow automatic fire.
If we look at vz. 58, which is not AK, but that process of making civilian version from surplus gun is same in concept, looks like this:
That block of metal was added just for that reasson and in early days it caused some problems, over here known as "hermex effect" named after one of first companies who started with civilian vz. 58 - after some time, due reshaping or moving slightly, it turned gun in essentialy bolt action.
Sure over the time, know how was developed and new remades doesnt have that problem.
Point is its quite an impact into gun mechanism itself
Also suplus civilian vz. 58 was build from parts and ussually got new finish - in my case there was little bit more/less material on touch plate of wire stock, making it move under recoil, lower handguar also moves a little bit
Add cheap surplus chinese ammo from 70s, unskilled shooter and terrible (by mi opinion) iron sights and you got what you got.
And thats just guns from one manufacturer, one factory. AK sometimes had diffrent specs even inside one brand, countries
Also doesnt need AK to be imported in US as parts kits? If so, its another remade
AK is platform (again, vz. 58 isnt AK, just use it as example of similiar proces) thats used in +/- same extense as AR15 is. That would not happen if it would be bad design (but it also doesnt mean its best)
So to conclude it - surplus AK are few times remaded guns and ussually need some tunning. Store AR15 logic doesnt apply here (also AK is diffrent even by concept of use)
If you get newly made AK, its a diffrent story and you should get gun in quallity according to its price range
Yet, i think AK are overrated and more "pushed" to current status, but i think same about AR15