How good is 2....Nf6 Scandinavian with kingside fianchetto?

Black scores poorly with it, but here is a list of players that do play it:

Strong players who used this line as Black:

Gerald Bonner 

Yann Michael Guidez 

Vladimir L Stoockalov 

Martin Breutigam 

Elena Luminita Cosma 

3...Qd6 might be slightly better if U know how to play it.

 Btw, 2...Nf6 is not worse than 2...Qxd5.

The whole Scandinavian Defense is kinda an iffy opening in my opinion. However, it frankly doesn't matter much if you're not a strong master. 

2....Nf6 is very playable, but not so much with the kingside fianchetto.

The best set up is the Matthias Wahls Set Up with Qa5. 1. e4 d5 2.exd Qxd 3.Nc3 Qa5 Forget everything else. But I would recommend you learn the French as well to be able to play something else, just in case. This opening is completely correct. I checked it with a strong chess computer. But you must read the book and study all variations.

Yeah, sure, U can even play 3...Qd8 back without any problem below the strong GM level.

Correct. Although I have to say it's probably quite tough to win as black in those lines, especially the 6. h3 lines.

even Carlsen played Qd8 to beat Caruana some years ago so all is playable.

For sure, after 1.e4 d5 2.exd5 Nxd5, one of black's main ideas is to attack white centre (White usually continue with d4-c4), so developing with g6 makes sense to pressurize d4, with Bg4.

Just develop your pieces, play logical/natural moves, and at your level, nothing bad (almost) will happen.

Qd6 scandinavian, Caro Kann (which one), are all different (though leading to similar pawn structure), so trying to compare them is useless.

GM David Smerdon published his book "Smerdon's Scandinavian" just over 2 years ago.  It has many ideas to prop up the 2... Nf6 lines of the Scandinavian.  It poses a lot of practical problems for white to solve, although it requires very bold play on the part of the black player.  As a long time Scandinavian player, I can't vouch for it's soundess at the highest levels, but it's great below master level, at least.

Smerdon is from Australia, as is GM Ian Rogers, a long-time player of the 2.... Qxd5 lines.

I've played the 2...Nf6 stuff quite a lot. If white knows theory then the 2...Nf6 lines can be really tough for black. I don't play it at tournaments anymore. Qxd5 is objectively better.

You could work hard learning many lines, and gaining experience with 2...Nf6 and probably have ok results up to a really high level... but I think that time is better spent elsewhere.

Caro is possibly a good low theory substitute for Scandinavian, but it's a different opening like poucin says.

Here is a nice video series by FM Santagati explaining how White can play against the Scandinavian 2...Nf6 among others. The problem after Black recaptures with the knight on d5 is that gives White a strong initiative on the Queenside when the kingside fianchetto does not offer any kind of strategicall goal for Black. Many tried to play f5-e5-e4 but it appears Whites attack consolidates faster on the Queenside due to the initial position after Nxd5-c4.

The video has a lot of generalization with out getting in many detailed variations but its not far from proving an advantage for White.

https://youtu.be/M4Umy8XzHYo

oh, i see you sent the bishop pair ghost back to oblivion

ohhh, Antiparos, how nice... i remember playing chess there sitting inside the sea   

and... other stories

Is just a terrible move.