This may sound simplistic, but one of the main issues (with any piece placement really) is how much it's blocked by friendly pieces. Blocked by enemy pieces (even a solid pawn chain) isn't always so bad, as it can be broken down, or even sacrificed against. A friendly unit is an invincible obstruction.
So this is the main problem with a double fianchetto. Not only do the knight's most natural development squares block them, but any pawn in the center will too.
Many openings leave you with 1 knight or bishop that will need a few moves to improve, or will need to wait until a later stage in the game to realize its full potential, but with a double finachetto you'll often be stuck with 2 (or more) minor pieces that need definite improvement.
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You posted an opening that shows a very active fianchettoed bishop, lets look at a counter example from the kings indian defense.
Black has practically forced this central pawn structure to block his bishop, what's more is he put his knight on f6 too, so what's going on?
It's ok to have 1 minor piece in a defensive role. The bishop helps shore up the kingside dark squares and adds some influence to the center all while not getting in any other pieces way. The c8 bishop hasn't moved, but as with many e4-e5 structures that involve kingside attacking ideas, it's pretty good on its home square, and the knights can dance towards squares like h5 and f4.
All this to say, the health of the finachetto is linked to the health of the position in general. If your other 3 minor pieces are not badly placed, then even a fairly passive finachetto can be good.
Recently I have been very interested in the question of whether a fianchetto of a bishop (or both of them) is a good idea.
I understand that different openings lead to different positions, which in turn have different requirements (such as whether the player should fianchetto a bishop or not).
Similarly, a fianchettoed bishop can have pros and cons in a position. On one hand, a fianchettoed bishop can give greater control of a long diagonal. Conversely, if a fianchettoed bishop is lost, this can lead to weak squares of one colour or another (especially if the opposing bishop is still on the board).
Because of this I am not really set on whether I should play a fianchetto. An example of this comes in my learning of the Philidor defence.
The Larsen variation (above) con often be played with a fianchettoed Bishop.
Alternatively, a variation of the Philidor including Bc4 (or other variations such as the Antoshin lines) are better played with short development of the kingside bishop.
These are only two lines of one opening, with clear differences between them.
What are your opinions on whether a bishop should be fianchettoed?