Ng5 - how bad is it?

Dababy that isn't enough theory
idk why cc decided to mess with the variations but here theory dump with slight annotation

Ng4 Bf4...? Poor mans Budapest gambit
It's basically just a Rubinstein Budapest with stuff in the middle

It's a completely fine move. It's important to note that it forces a response from black, as the threat of nxf7 is too great to ignore. There are a few variations, but anything besides the main move na5 is slightly better for white, and even the main line provides white with an extra pawn (Albeit an undeveloped position).
Ng5 has been the "mainline" for literally centuries. Many GMs believe it is fine, but personally I'm with a select few who believe 4. Ng5?! is actually dubious. I'd prefer to play the open variation 4. d4 with White. As for Black, I prefer the Polerio Defense and believe that the sharp lines where Black sacrifices 1, 2 (or even 3 sometimes!) pawns actually "refutes" this entire opening. I've studied a ton of theory into this line out of interest and it is very sharp and complex, but I believe White gets the slightly worse position out of it all.
With this said, if I play the Polerio Defense and lose, then it is probably because my dynamic compensation for the material (development, attacking chances, initiative etc.) fizzled out (and dies in the endgame), but that is probably because I played sub-optimally at some point.
Summary: Ng5 is probably okay and certainly playable...but perhaps one day I might prove some "refutation" that conveys Ng5?! being innaccurate.
Wheres @KeSetoKaibe show the two knights line that you play