On shield, lasers have 7 frames of stun. When they connect, they have 8 frames of stun. They also have 4 frames of hitlag.
Because Falco's lasers autocancel, we're just going to ignore the existence of hitlag because landing when they connect means that Falco is actionable right when the stun begins. Falco has 5 frames of jumpsquat and his nair is active on frame 4. Assuming that the Falco jumps and nairs as quickly as possible, it takes 9 frames to come out. That basically 100% ensures that he'll ALWAYS have the advantage in that situation, even with a few frames of leniency for imperfect inputs or slightly higher lasers.
I don't think it's quite as black and white as that. First, and perhaps most importantly, Marth can shield grab an instant nair (assuming it hits on the first frame). The Falco must space around Marth's grab, which means he has to land behind or significantly in front of Marth. Shine will only connect if Falco spaces behind your grab. In addition, Marth is actionable for 2 frames before the first hit of nair comes out. Falco's nair has a max of 7 frames of shield stun iirc.
So what does this all mean? It means that Marth can chose different options based on Falco's spacing that beat the pressure. If Falco...
instantly nairs at any spacing
Then you can always wd (or maybe roll??) away. The nair hitbox will be too high to catch crouching marth. I'm pretty sure this is impossible on reaction though, so read on if you can't read Falco.
hits shield with first frame of nair and lands in your grab range
Then grab him
hits shield with first frame of nair and lands spaced in front of you
Then do any oos option. It takes 7 frames for Marth to be able to act after he blocks the nair. Falco, on the other hand, must land and go through 7 frames of landing lag, giving Marth a huge frame advantage. This is compounded by the fact the shine won't connect (though I think if their spacing is gdlk, jab will outrange grab, though Marth has a number of ways to deal with spaced jab on shield). Good options: wd back, jump back or to a platform, fair, roll.
hits shield with first frame of nair and lands spaced behind you
Then wd forward, roll, up b, or dair. You have to be more careful here because now shine will connect, but even still you have crazy frame advantage.
instantly nairs, but spaces so it doesn't hit shield on the first frame
Then you must react to the timing/spacing or roll. The longer it takes to hit shield, the longer you have to act BEFORE getting hit. This of course comes with the trade off of having less time to act after getting hit. So you need to be able to decide if the Falco is giving you enough time to do an oos option, or if the best bet is to still get hit and attempt to escape during the period Falco is landing. Alternatively, roll is invincible on frame 4, so it should get you out before he can hit you.
delays nair
Same as above