I received it today. First impressions:

POSITIVES:

1) Quality construction materials and packaging, as expected for a PSU with platinum certification.
2) Cable set with many options! (for GPU, CPU, etc.)
3) Hybrid function to operate fanless and be completely silent. Even in normal mode, the noise level is very low.
4) 10-year warranty from Seasonic with registration on their website. Very simple process.

NEGATIVES:

1) The cables are NOT sleeved and are very rigid, a beginner may struggle with cable management.

17/12/2020 - Perfect so far. I use it in hybrid mode, meaning fanless (the fan only activates automatically when needed, and so far I haven't pushed it too hard). Simply excellent.

11/01/2022 - Absolutely no issues. The PC runs at least 8 hours a day and at least 3-4 hours in gaming sessions or rendering 3D models. Seasonic is the definition of "you get what you pay for" when it comes to power supply.

14/11/2023 - Now the power supply has taken over my new PC, with Ryzen 7000 series and Radeon RX 7800 XT 16GB GPU, and it continues to provide the required power flawlessly! The definition of a good purchase, Seasonic with closed eyes.

I will start with the noise*. Up to 600w and room temperature of 30c, the fan does not exceed 1000 RPM, which practically makes it silent.
Also, it does not have coil whine. I was a bit worried because some gold and titanium ones had it, but I couldn't find anything about platinum ones from Seasonic.
Personally, I decided not to disable the fan at low speeds because at 450 RPM, it doesn't have any audible difference compared to being off. The funny thing is that I bought it for passive operation, but ended up not using it.

As for electrical noise, from measurements I made with a digital oscilloscope when the system consumed 220w, I found 26mVpp ripple at 12v, 45mVpp ripple at 5v, and 33mVpp at 3v3. The 12v measurement is incredibly low, while the 5v measurement, although within ATX specifications, seems a bit high and contradicts the report from Cybenetics that I will mention below. Since it is a system measurement, it is possible that some component is causing the noise during operation, so the cause cannot be isolated to the power supply. However, it was expected to observe noise at 12v, as the VRMs of the processor and graphics card consume large amounts of power. If I measure the 5v disconnected from the system, then I have 25mVpp.

The positive thing about such stable power supply is that the processor is stable with less current. Specifically, with a Ryzen 3600 at 4.05GHz on CCX0 and 4.025GHz on CCX, I lowered the voltage from 1.2V to 1.1825V, resulting in 5W less power consumption and 2 degrees Celsius lower temperature. The total system power consumption dropped from 255W with the previous power supply to 223W or 218W with the voltage reduction on the processor using this one.
Also, it should be noted here that the previous power supply was not some "cheap" one.
I was using the Enermax FMA II 535w for the past 14 years! (purchased in 2006 for 80€)
In 14 years, it is the only component that remained common in all 4 computers I have built so far. In other words, it more than made up for its cost, something I expect from the SSR-850PX as well.

For those looking for a detailed review of this power supply, check here: ftp://members.cybenetics.report/PDF_Reports/d/cybenetics_ehG.pdf
I believe it will be very useful as it was not easy to find a review, and this report played a crucial role in my decision to make the purchase.

Purchase price: 173€

*Greater efficiency, hence lower thermal losses, resulting in less need for cooling and as a consequence, less noise. The lower energy consumption is not a significant reason, as in Greece we have cheap electricity compared to other countries, and the premium one pays for platinum over gold or bronze takes years to pay off.