I am no expert, but from what I understand, heat is the issue with the logic boards, or was with mine. The chipset is a BGA chip with solder balls/pads on the bottom of it instead of pins. With the am out of heat the g5 generates, the solder joints on the chip start to soften and flex, some even fracture under the stresses. Because of the location of the ram slots being close to the chipset, and nothing under the slots to support them, the logic board flexes quite a bit when users install memory with a lot of force. The flexing causes the BGA connections to seperate from the board and you get a dead Mac. Baking the logic board has worked for me to revive it.
On the case of the psu dying, it could very well likely be caps going bad for the same reason of heat or underrated caps, but you could have bad mosfets, diodes and fuses just as easily.
Also, the G5s solder issues, I feel, were primarily caused by the ROHS compliant solder used. I don't thing I have any of it left in my tool box even, after I have seen all the crap it causes.
atx supplies I work on always seem to have at least one mosfet burned up.
