I don't know why anyone would be afraid of the gambit lines in the Advance. If I could have that position as black in every game, I'd be very, very happy. All you have to know is a little theory and it helps to have some practice in the line. But in general, ThrillerFan is right: all the alternatives to 6. a3 are bad, and black is at least equal and possibly better (especially if white gambits a pawn). To be fair, though, the Advance is obviously not the acid test of the French Defense, and will never be considered as testing as 3. Nc3, for obvious reasons.
What are the obvious reasons?
Well, firstly, conceptually it is a bit flawed. It releases the pressure too early without forcing a concession first, like winning the dark bishop in the Winawer or forcing Nfd7+being able to play f4 in the Classical Steinitz.
And if you need other reasons, just check out the statistics and the number of GM-level players who play the Advance as opposed to Nc3. You'll find a trend.
I don't know why anyone would be afraid of the gambit lines in the Advance. If I could have that position as black in every game, I'd be very, very happy. All you have to know is a little theory and it helps to have some practice in the line. But in general, ThrillerFan is right: all the alternatives to 6. a3 are bad, and black is at least equal and possibly better (especially if white gambits a pawn). To be fair, though, the Advance is obviously not the acid test of the French Defense, and will never be considered as testing as 3. Nc3, for obvious reasons.