Do PSUs all simply fail if driven too hard? If there some type of breaker or fuse that trips on some models if an over-current situation occurs?
OCP (overcurrent protection) is a common feature in PSUs, however it does not function as I think you are implying; it protects components from current spikes.
There are however also overload protectors, which are essentially fuses, that kick in to protect against draw that is too high on most PSUs.
This is why you rarely see an explosive failure in any quality PSU, even at high draws. If you buy a POS like the OP, well... then who knows what will happen. A quality PSU however can be run at it's full rating 24/7 365 days a year, and it won't bitch. Look for PSUs that are rated for their supply continuous rather than peak wattage.
As I mentioned before, I ran a 750W PSU, with ~875W coming from the wall (4 or 5 heavily overclocked 5870s), which was something like 5% above its rated capacity, and it kept ticking even after weeks of this until I swapped it out for a beefier supply.
I suppose the market needs people who refuse to listen to good advice though, or else all the cutrate vendors would go out of business.
I can see that my choice of PSU (upon the recommendation of multiple established miners on this forum), has clearly upset you. If I had known you were going to be this distraught I might have asked if you wanted to chip in an extra $50 for me to get a single rail PSU. So I deeply apologize for upsetting you so deeply.