I don't think I agree with the bed of nails thing, for the most part. But I think it does make sense (and I think this is certainly the case with Trump) that someone who is more scandal-prone thus has much more experience talking their way out of them, and thus a scandal that could bring them down would have to be bigger than it otherwise might be, and also leading on from that, someone who is not that scandal-prone will be less likely to know what to do when one comes up.
But I also think a key part of that would be that it's not just being accused of things, it's actually having to deal with them. And I think that might be something that could hurt Sanders - he's never really been called out on a lot of these potential scandals, so I think actually all of that stuff coming out at once would probably hurt him at once.
There's also the question of how prepared someone is to deal with scandal. I think in 2016 something that hurt Clinton was less the scandals inherently and more that she wasn't prepared for them (partly because a lot of them were bullshit), whereas Trump knew the things he'd done and thus what he'd be accused of and thus how to talk his way out of it. As an example in the upcoming election, I think the Harris nepotism sex story has been floating around long enough that she will have prepared a response for if and when it comes up (and if she actually hasn't, that lack of aforethought would be more of a deal-breaker for me than the scandal itself). Lastly, some people might just be really piss-poor at dealing with scandals and elongate their time in the public sphere and somehow make them even more embarrassing entirely on their own (I'm referring, of course, to Elizabeth Warren, who seems absolutely determined to remind everyone as much as possible about that time she pretended to be Native American).