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Redditor finds heavy block of iron shavings inside cheap PSU, also appears to lack safety protections
(www.tomshardware.com)
This is a most excellent place for technology news and articles.
I think we should be fair and give credit where it's due, that advice may have been going around but more likely in reverse form – "if a PSU is very light something's wrong". Any gamer with half a brain has long since learned to buy PSU's based on reviews coming from reputable testing labs. There have been such labs available for a long time now, jonnyguru.com (Jean-Claude Gerow) started doing detailed PSU analysis around 2006 I believe.
To me this is the most important reason for building your own PC. If you don't care or don't want to research each part then sure, get a prebuilt. Otherwise, it's really nice to know what's in it and do your research on each piece so you know it's quality and will be supported.
Warranty is the biggest reason for a prebuilt. Anything goes wrong with it and you're not spending money on things to test and experiment with. You send it in, it comes back working.
You get warranty for parts too. Unless you meant warranty as a substitute for building know-how.
It's a convenience factor I think. Send the whole thing away and it comes back working. Opposed to having to find the faulting hardware and determining the type of fault and dealing with the vendor for that specific part in hopes that it's actually the issue.
I don't personally view that as a convenience but understand the sentiment. If my PSU died, or something similar, and I had to send my entire machine just to get it fixed, that translates into working downtime for me.
It's nice to just have some spare parts or your old parts to swap into temporarily while you rma the dead part. Of course, this assumes that you can do a bit of hardware troubleshooting (which I admit isn't something most laypeople can do).