Ng5 - how bad is it?

Sometimes the chess.com engine will call it’s own moves inaccurate or even mistakes

Chessable has a great course on the Italian that covers 4. Ng5 against 3 … Nf6 and the evans against 3 … Bf5. I believe it is also on sale https://www.chessable.com/the-dynamic-italian-game/course/42964/

I’m going through it right now and there is also a section that just covers the most common lines to get you started. It also has sidelines as early as after 3. Bf4 like 3… h6, 3… d6, and others.