Spaced bair doesn't work on anyone. It's not an approach. You can drop shield and attack during the inactive frames, or just shield through the whole thing and let it push you back, or retreat and bair through the inactive frames.
Spaced nairs are easily punishable if used as an approach, they're only safe as a retreat.
It works pretty darn well if you're that friend of mine who doesn't just SH spam it like an idiot. And what are you doing to punish it with? Jab? Oh right, you can Jab cancel, except the problem is Luigi's Jab cancel options all have much shorter range than his Jab, so depending on how far you slide/how much he doesn't suck at spacing, landing a combo gets pretty iffy.
And now just to be emo, let's think for a moment that this actually is a staple strategy. After all, the success of Luigi's Jab cancel does partly rely on neutral DI and interrupting reaction time.
G&W's N-air is safe on block, and excellent in crossovers. If he's keeping you guessing as to what move he's going to approach with, it's a pain in the butt to consistently punish him.
That wasn't even the worst of it, considering how hard it is to get around his F-air and D-tilt. =(
You're wrong about Bair in the Marth matchup. Fair works amazingly as Peach and has similar properties. You should use it to punish missed fairs. The ending lag doesn't matter if it hits.
What Marths are you playing? Because frankly, I honestly have had pretty little success spacing with B-air against Marths that don't throw out attacks randomly and let themselves get baited easily. I mean...if he just waits and just scouts your options with SHFFs, what are you doing to create an opening? B-air is much too punishable to be worth it except in combos and edgeguarding from my experience.