To be honest, I had tried this scenario solitaire several times and Athens was winning in 2/3 of the situations without any possibilities of Syracuse to react.
Now that I have played face to face, I am happy to have confirmed that both sides can win.
Luck on Sea crossing is one major factor of victory.
Number of battles and siege are also limited, so victory or defeat are linked to some important dice roll more than in the other scenarii.
I found the major issue for the Athenians was Winter Quarters. The sea crossing has never been a problem. The Syracuse cavalry has always kicked Athenian butt