It could be that these machines have weak power supplies to begin with, or that they aged particularly bad.
The friend with a G4's power supply failed spectacularly while it was still under AppleCare, or very shortly thereafter. He replaced it, and it has, as I'm understanding, been running solidly for the past decade.
My Blue-And-White probably still turns on -- I should go pull it out and test it, because I happen to have it here in my house -- but I don't know what's on it or necessarily have time to do a lot of reinstallation. The good news is that I have 8.6 and 9.2.2 CDs hanging around, the bad news is that I don't know the state of its hard disk.
When this all happened, my blue-and-white was already a little old. I'm thinking this was in 2005 or so, so it was six years old and that's pretty much beyond the normal professional lifespan of a desktop computer. Even (especially) one from about that time. I also didn't get it new, so I don't know if anything happened to it previously to weaken the power supply. I know that the original motherboard was at one point replaced with one from a Yikes! Power Macintosh G4, but I don't have any reason to believe the power supply was swapped at that time.
It could be that my unit's power supply suffered some kind of undue stress, perhaps something that itself caused the motherboard to fail, but other than that one USB card (which didn't cause any problems for the Dell PC I put it in) it performed admirably.
I'll stress again that I really don't have enough data about that one card to formally accuse it of anything.
The only real data I have is that...
I had the card, ran it in my blue-and-white G3 with a Yikes motherboard until the machine stopped turning on. I removed the card, pushed buttons to no avail, until randomly it booted up a few days later. PUt the card back in, same thing. I eventually stopped putting that card back in, and I eventually also stopped having problems with the system.
My friend had the card, ran it in his MDD G4 until his power supply caught fire and/or exploded massively. He put a new PSU in the system, and if I'm remembering correctly, put the card back in, and it has run fine since.
To add to what rsolberg says though, the ADC monitors, especially the 17-inch CRT ones, are said to have caused problems for the power supplies of those G4 systems. That's sawtooth or gig-eth and newer stuff, though.