Looking at the way it's programmed to switch in between cores, these CPU's are fragile. It's just not all about thermal management, but pretty much degradation. Look at 2x00 series. Many reviewers just boot a 1.4V into a CPU and hit a OC on that. The 1.4V is seen to degrade the CPU in just months (!).
Is that really the case though? All I've seen are a handful of people saying things like "it's been proven." But I see many more people running them around or even a little beyond 1.4... with very few, if any definitive reports of degradation. I'd think if they were really so fragile we'd have more concrete reports of chips dying off. But if they're out there, I can't find em! Only a few sporadic accounts from people who think that's what happened to them. Which, maybe it did! But it reminds me of the RTX memory failure issue. Something that people say is a thing that's happening a lot, when the real number was probably still pretty low, and it was just one big, freak occurrence.
I could also go after the claim of switching cores being a sign of known fragility. It could just as easily be thermal and there's no logical reason to go one way or the other, because the longer a core is loaded down, the hotter it gets. It's one of those nebulous things. If you want it to be for degradation, there's a rationale. If you want it to be temperatures, there's also a rationale. Nobody but AMD really knows.
Reviewers pumping 1.4v in for their OC testing and calling it a day... yeah, not a fan of that myself. Probably encouraging a good chunk of less experienced builders to run way higher voltage than they need. 1.4 is too much, and there's a good chance, at least with the 12nm and 14nm, that you will see eventual degradation. That's undeniable. AMD itself has recommended not going over 1.35 for a continuous-use all-core, IIRC.
Whether that makes them weak or unreliable, I'm not sure. How many modern CPU's are really expected to run at 1.4v continuously? I'd think that most people with some experience from either side of the fence would say that's pretty high.
That aside, the way Ryzen 3000 currently jumps from core to core, as far as anybody can tell right now, isn't quiiite the intended behavior. I'd say the reason for what they're doing with this boost stuff it is to squeeze the max performance out for the least power. I'm not talking theory, here. It's just my conclusion looking at what's going down and what the outcome is. I mean, I have played with a couple handfuls of these Ryzen chips, now. All generations. I'm not an expert and don't know nearly as much as many of the regulars here, but from what I've seen a MAX (as in, approaching or fully hitting unsafe voltage) all-core OC is pretty much always just a *little* inferior in performance to an X model attaining max boost will pull off, for less power. For instance, I have a 2600 that will do an all-core of 4.2 steady.. at around 1.3v. I can even push it past that if I'm willing to go to ~1.4v... BUT, even doing that, it never benched as high as its X-model sibling and the thermals were just impossible. That seems to be how it goes, and is a big part of the prevailing wisdom that it's best to let it boost naturally. And that's still the consensus, because it just performs better.
So I see what you're saying - you can easily cook a Ryzen running the voltage too high, but I'm much more willing to believe it's for max performance and thermals primarily, as they actually do run better when you run them the way AMD wants them.
I don't get why people get up in arms about this stuff. It's annoying, sure, but at this point it's to be expected. AMD's boost system is both very advanced and very young. It's going to be a while before they get it right. In the meantime, the Ryzen 3000 line still has some of the best-performing CPU's on the market. It's not like they're completely broken by these little quirks. It's nothing like the whole bulldozer situation where people completely didn't get what they thought they were getting and the performance wasn't what it should've been. You're still getting a good, working CPU for a good price - it just boosts funny lol.