So there's a sharp system you like very much as Black. Why not play it reversed as White with an extra tempo? How useful would that be in the critical lines? If you're attacking the kings on opposing wings, say, an extra tempo might be very useful.
But critical lines are not the issue. What you need to think about is the non-critical lines, especially those which nobody plays because they offer White nothing and merely cede equality straight away. That's the drawback. In the reversed opening Black doesn't require an advantage. Equality will do just fine, thanks. And what are you going to do then as White? The game below is a case in point.
I wrote about the Scotch: Steinitz Variation (4...Qh4) in Game 19. The critical lines here involve throwing a knight into b5 with either 5 Nc3 Bb4 6 Be2 Qxe4+ 7 Nb5 or 5 Nb5 at once. Other ideas, such as 5 Nf3 or 5 Qd3 or 5 Nc3 Bb4 6 Qd3, allow Black a good position. A reversed version arises in Mengarini's Opening after 2 Nc3 Nf6 3 a3!? d5 4 exd5 Nxd5 5 Qh5. As you'll quickly realize, the extra a2-a3 rules out any ...Nb4 ideas here, so White can look forward to a “good position” in one of the less critical lines.
However, there was one sequence I'd forgotten about: 4...Qh4 5 Nc3 Bb4 6 Qd3 Nf6 and now 7 Nf5!?. In all my books, GM Lev Gutman's 4...Qh4 in the Scotch Game is the only one even to mention 7 Nf5.
As Gutman shows, White can ignore the attack on c3 after 7...Qxe4+ 8 Qxe4+ Nxe4, because 9 Nxg7+ Kf8 10 Bh6 Nxc3 11 Nh5+ and 12 Bg7 then forks c3 and h8. I'd since looked at this myself and concluded that Black's best response is to target the h1-rook in turn with 11...Ke7 (or 11...Ke8) 12 Bg7 Ne4+ 13 c3 Bc5, when 14 Bxh8 Nf2+ 15 Rg1 Ng4 16 Rh1 Nf2 leads to a draw by repetition. All well and good. Black is fine.
But how does a2-a3 help White in the reversed position? Answer: It doesn't. White has to take the draw in the same way. So when my opponent answered 5 Qh5 with 5...Nc6 6 Bb5 Qd6 7 Nf3 Nf4!, I stopped short and thought: “Bollocks!”. No further comment required.