Some of Seasonic's units are, but just like Corsair, EVGA etc they have some real turds as well.
I don't know if Seasonic manufactures most of Corsair's units. Corsair CX and RMX PSU's are made by Channel Well Technology. I haven't looked into the rest of their product line.
The Seasonic S12III isn't a Seasonic unit. Manufacturing of that product line is outsourced to Shenzhen Rui Shengyuan Technology from what little information I could find.
This review:
https://aphnetworks.com/reports/seasonic-s12iii-500w/3
Mentions that the 500W model has no OTP (over-temperature protection) or OCP (over-current protection).
The S12III 450W and 500W units also seem to be group regulated just like the previous S12II lineup (although there aren't enough in-depth reviews to 100% confirm this). Group regulated PSU's were bad 10 years ago and terrible now. I wouldn't put these anywhere near any modern system, that's just asking for trouble. Group regulated PSU's can't compensate for voltage fluctuations on a single rail which, depending on your system and what kind of load you're putting on it, can cause voltages to run out of spec and crash the system or even cause hardware damage.
Is it an issue? Maybe. Can it be an issue? Yes. Is it an unnecessary potential issue? Absolutely yes. Just buy a PSU for 5-10$ more that isn't trash tier.
I don't particularly disagree with the performance aspect, the difference is negligible. However, if you don't already have a CPU cooler that fits the 3600 you might consider a 3600X as a 3600 + decent cheap CPU cooler is basically the same price.
Personally I recommend a 3600 + CPU cooler but that seems to make some people see red as the 3600 comes with a cooler that is "good enough". It really isn't in my opinion. I have one right next to me from when I did my build a few weeks ago and it's a flimsy little thing. Plugged it in just to test and it was horribly loud, unplugged it very quickly. It's pretty much on the same level as Intel's box cooler.
It matters to precisely average people. Maybe you're lucky and you have no issues with a bad PSU, that still doesn't make it a good buy.
A bad PSU can cause problems in so many different ways and all of them are hard to troubleshoot. Problems can even take a long time to manifest while the PSU has been slowly degrading the other electronics in the system.