I suggest you actually build that Flak emplacement and see how Amazing it is, and tell us again.
Of course it's easy to decrew and use against.
But combined that with good environemnt and wires - to protect from nades and approaching infantry - and then it's quite good
At least I had good use of them whenever I decrewed it and used agasint OKW with backup of MG.
And where I said flak is amazing? Point me please. It's useful, good but amazing