Modern ATX PSU are not so accurate regarding the 5V and 12V as that is mostly used for legacy"mechanics", spinning disks etc, and not so much for the motherboard electronics. Modern PC mobos run on 3.3V, so that's where they output "best quality" voltage.
Or in other words... avoid using modern ATX PSUs for Amiga, and instead go for dedicated new Amiga PSUs (there are quite a manufacturers), or all around good quality general purpose PSUs like the MeanWell ones.
