I used to think that the Heralds were just super powered KR's but the WoB's have kind of flipped that idea on it's head for me. I don't think that they are super crazy over powered though. I would like to think that If you where able to pin one down you could overwhelm them to the point of death. I don't see a Herald surviving a severed head. I know we see Renarin tank the TC step but isn't he kind of an edge case? We have no idea what is going on with him as far as what his powers do and how they are changed by the corruption of his spren. Maybe the TC just had a really high arch?
Do we know if the Heralds need to be holding the Honor Blades to access the surges? Do we know if they bonded the blades like we see Szeth do later with the Honor Blade or is it possible they didn't know how to use a gem to do that? I would assume that they were bonded to Heralds but I don't recall seeing one of them ever summoning one and we do know that Taln's was stolen... which really begs the question "how does one steal a bonded Honor Blade?". It's interesting to think about.
Also, it seems like people want to think of the Heralds like super powered mini gods fueled with infinite power, which they are, but they are also just men. They must have been really tired of dealing with all the crap. I mean think about it. You get sent to Roshar for the Apocalypse, spend what is probably months doing nothing but fighting, every day. IF YOU LIVE you have to be sent to Braise any way, where you will be hunted down like an animal and then once caught you will be tortured. Lets not forget that the Oath Pack allowed them to share the burden of torture with each other, so you don't even have to be the one being tortured to be tortured. This torture can go on for hundreds of years because you can't die. And then some one breaks. And you get sent back for the Apocalypse. AGAIN. This is a really bad deal.
I wouldn't be surprised in the slightest if some times the Heralds were killed because they just simply gave up.