I strongly disagree. Marth certainly gets punished harder than spacies. While a chaingrab on fox or falco at FD might be deadly, that's about it. On the other hand, one dair shine from a good Falco or one Upsmash at 60% from Fox can lead to the stock. Even when Marth survives, he's in a much worse position than the other high tiers because he's floaty and light. One bair will kill him, a well placed f-tilt will kill him. Unlike Peach and Puff who are also floaty and light, he actually dies easily and doesn't have the advantage of an amazing recovery. In this sense, Marth is absolutely punished harder than spacies. Falcon is a less solid character overall than Marth, but I would say in terms of being punished his weight actually helps him survive more at times.
i think that it's stretching it a little to say that marth gets punished harder than spacies. However, if you limited it to only the small stages, and defined punishment as likelihood of scoring a stock, then I would agree that fox punishes marth as hard as marth punishes him. Fox can get as much off a conversion on average as marth gets off his guaranteed combos. A good spacey should be able to follow up with a potential death mixup on almost any hit after 40. The problem is that a lot of spacies are bad and play for damage for some reason. Add on the fact that mixups are worse than consistent damage for getting through tournaments and I wouldn't want to be going fox vs marth in tourney. But it's still doable. Anyway this whole question is really ill-defined. I mean, what constitutes punishment. If you don't consider fox's 2 hit into pressure that is likely to lead to another two hits as part of the first hit's punishment, then of course marth is infinitely better than fox.
also, spaceys can DI in such a way to trade damage for survival. You see all these flashy combos that result in death and don't understand why marths don't just always do that. Marth doesn't have these guaranteed kill setups. He just completely utterly wrecks spaceys who do not know the correct way to DI or miss a DI. marth's legitimate platform combos are all like that m2k vs leffen combo on FD, massive amount of damage, but no finish. though on platform stages, i think most of the time you are better off sending them offstage in such a way that you minimize their recovery options to one or two things and then picking up kills that way instead of getting them to 110 and then having them make some comeback.
however, there is no way you can compare marth's survivability on PS and FD to fox's. He eats some silly combo and just DIs out after a few hits. Fox tries to DI out and still eats 50+ percent.
I do feel like marth's survivability is barely better than falcon's. falcon stays on stage easier but once put off dies, while marth gets sent off stage easily but has a chance of living afterward without the opponent messing up. Why are we debating this though? is it really worth arguing over levels of trash recovery? Marth is at high risk of dying past 70 when sent off stage by jiggs, peach, and sheik. And against fox and falco if they are good they have mixups on you that can result in your death like half the time. Luckily most foxes don't know any of the mixups so you get away with it.
However, vs spacies I would have to say falcon has it worse. Marth will DI out and then have some mixup of getting back on stage. falcon will DI out and just die, and fox can kill him at ridiculously low percent and has combo setups that put him off stage. Marth always has options off fox's early setups unless he DIs a shine poorly.
But against peach, sheik, and jiggs, I would put marth's survivability on par with falcons.
Falcon is the reason that I still play marth for fun when I'm not playing peach. Cause whenever I start to think I have it bad, I remember falcon and his eternal fight for freedom from spacey dominance. Uggh, just thinking of all the guaranteed falcon deaths i've seen with the falcon DIing perfectly, I feel bad for him. Poor guy
edit: wanted to disagree in part with ado's post. Marth may not have high variance in all his matchups. but vs fox and falcon's dashdance he doesn't have good low committal options except for retreating moves and dtilt, both of which are limited on small stages. Consistent options with low committal (low committal being defined as a wide pool of followups when wrong) are of course more available against slower characters. it's a rather limited problem, but there are lots of foxes and they don't typically play FD in their set first match.
however, looking back, I guess overall I would agree with the statement since I think against the non top six marth has very low variance if you do the matchups correctly.