user-1: The Muzio Gambit is -0.8 by Stockfish depth 22. However, if the f7-pawn is moved to the g-file, the evaluation falls to -3.5. How is that so? Both positions seem  fine for White.
[FEN ""]
[Title "Muzio Gambit"]

1. e4 e5 2. f4 exf4 3. Nf3 g5 4. Bc4 g4 5. O-O!


user2494: In the Muzio proper, White doesn't have a threat other than 6. Qxf4 (which doesn't mean so much, because that's what the c1-bishop wants).  Shift the f7-pawn to g7, now White's threatening Qf3-h5+, and that's hard to deal with.

user15223: One of Black's biggest problems in the diagram position is that White will get in Qxf3, Qxf4, followed by Qxf7+. With the f7-pawn on g7, Black can shut this plan down with ...g5. It's true putting the f7-pawn on g7 exposes Black's king to Qh5+, but it's not a big deal: Black can meet Qxf3 with ...Qf6, then Qh5+ with ...Kd8 or simply ...g6, and White has no followup. Meanwhile, Black is up a full piece.

user16650: As for "both positions seem fine for White": well, Black has a significant material advantage after 5...gxf3, so White has to find some compensation. If you don't have a concrete continuation which demonstrates that compensation, I don't think there's any reason to think the engine evaluation is confused.
In both positions, a sensible continuation is 5... gxf3; 6 Qxf3 Qf6; 7 d3. Now in the original position Black has doubled f-pawns and a bad pawn structure; in particular the pawn on f4 can't be supported effectively; when it falls Black will have two isolated pawns. Black stands better but has some weakness that White can try to exploit. In the position with the pawn moved from f7 to g7, Black could play 7... g5 immediately, or alternatively 7... Bc5+ and then 8... g5. Now Black's structure is more solid and White doesn't have anything like enough to justify the material disadvantage.

-1: <p>The Muzio Gambit is -0.8 by Stockfish depth 22. However, if the f7-pawn is moved to the g-file, the evaluation falls to -3.5. How is that so? Both positions seem  fine for White.</p>
<pre><code>[FEN &quot;&quot;]
[Title &quot;Muzio Gambit&quot;]

1. e4 e5 2. f4 exf4 3. Nf3 g5 4. Bc4 g4 5. O-O!
</code></pre>
 2494: <p>In the Muzio proper, White doesn't have a threat other than 6. Qxf4 (which doesn't mean so much, because that's what the c1-bishop wants).  Shift the f7-pawn to g7, now White's threatening Qf3-h5+, and that's hard to deal with.</p>
 15223: <p>One of Black's biggest problems in the diagram position is that White will get in Qxf3, Qxf4, followed by Qxf7+. With the f7-pawn on g7, Black can shut this plan down with ...g5. It's true putting the f7-pawn on g7 exposes Black's king to Qh5+, but it's not a big deal: Black can meet Qxf3 with ...Qf6, then Qh5+ with ...Kd8 or simply ...g6, and White has no followup. Meanwhile, Black is up a full piece.</p>
 16650: <p>As for &quot;both positions seem fine for White&quot;: well, Black has a significant material advantage after 5...gxf3, so White has to find some compensation. If you don't have a concrete continuation which demonstrates that compensation, I don't think there's any reason to think the engine evaluation is confused.</p>
<p>In both positions, a sensible continuation is 5... gxf3; 6 Qxf3 Qf6; 7 d3. Now in the original position Black has doubled f-pawns and a bad pawn structure; in particular the pawn on f4 can't be supported effectively; when it falls Black will have two isolated pawns. Black stands better but has some weakness that White can try to exploit. In the position with the pawn moved from f7 to g7, Black could play 7... g5 immediately, or alternatively 7... Bc5+ and then 8... g5. Now Black's structure is more solid and White doesn't have anything like enough to justify the material disadvantage.</p>
