After playing the Benko Gambit myself for a few years, and looking at the lines with Crafty, I think …Bxa6 in the Benko Gambit accepted deserves a ??, and yet …Nxa6, which I virtually never played, probably deserves a !!
This fianchetto variation by White kills the gambit in it’s tracks, IMHO, and it’s not just one line, all the main lines from there:
Look at move 15 of this position from the game above. ..e6 is the only logical break for Black, opening the position from a pawn down, whereas White is ready to absolutely pounce on it. Geller sacs the ..Rb4 because otherwise it’s connected passed a and b pawns after a …c4 push, which is really just a technical win (and I realize most class players probably aren’t good enough to convert this technical win even it is +2 for White, yes, but that’s not something to hang one’s hat on.)
For example White plays the important move b3, so that on …c5 there will be b4. Also Bh3 and Bb2 give the White bishops scope. However, with 5…Nxa6 with the idea of …Nc7 supporting an …e6 push, Black begins to get his center going, which is very important because that can turn into a good gambit for the activity.
Note that White allows the Ba6 to be traded with e3, which is not necessary (strategic error, IMHO), but allows the ..Nac6-c7 idea to be shown.
The Benko Gambit “feels good” because one can play 10 moves in 5 minutes as Black, but I think it only works (…Bxa6) because so many as White play it poorly or don’t know how to play it.