Disclaimer: I have not used any of the four SFX models mentioned in post #3.
Concerning reliability no one can answer your question. One thing PC equipment reviewers cannot do is assess long-term reliability especially with a statistically significant sample size. This isn't specific to PC power supply units, it pertains to pretty much all PC components (CPUs, fans, RAM, graphics cards, whatever).
If long-term reliability is a major concern, your best bet is to select a unit with an exceptionally long warranty. The manufacturer has the best idea of the durability of these based on prolonged laboratory tests in-house as well as RMAs.
Bill_Bright's points about the differences between Gold and Platinum are pertinent. The differences between the two are pretty minimal especially for consumer use:
Here’s what you actually get when opting for a Gold-rated PSU over a Bronze one.
www.pcworld.com
However one thing that hasn't been discussed is acoustics. SFX power supplies tend to be noisier than ATX power supplies -- particularly at higher loads -- because they use smaller fans to fit in the smaller SFX enclosure. A smaller fan needs to rotate at a faster speed to move the same volume of air. SFX power supplies are also narrower which means less exhaust vent space at the back panel compared to the wider ATX size.
What some SFX PSU manufacturers have done is to come up with the SFX-L form factor which makes the PSU longer. This allows the manufacturer to include a larger fan; this helps with acoustics. Naturally the PC case must have the space to accommodate SFX-L PSUs. The SFX-L form factor does not have an official specification for length so it is up to the buyer to do their due diligence in figuring out if a given SFX-L PSU model will fit in their particular case.
In my primary gaming build (see System Specs <==) I have a Silverstone SX800-LTI (800W, SFX-L format, 80 Plus Platinum) power supply. I didn't buy it for the Platinum certification. I bought it for its larger fan (120 mm) and the 800W capacity since it's powering a thirsty GeForce RTX 3080 Ti.
In an different build (NZXT H210 which is tight on space) I have a Corsair SF750 SFX power supply which has a smaller fan. In my specific usage case, this PSU isn't any noisier because it is a build with a lighter load. It's worth noting that Corsair offers a 7-year warranty. Originally the H210 case had an ATX power supply which was too bulky for the custom cooling loop that was trying to install at that time so I switched to the SFX unit to reclaim some real estate inside the case.
In selecting an SFX power supply, you really need to start with your usage case (what components it will be powering at what typical load amount) as well as the computer case dimensions itself. Shoehorning an SFX-L PSU might not be desirable in a small chassis although it might allow for better acoustics if used at a high load.
Naturally you pay a premium for an high-wattage SFX or SFX-L power supply versus an equal wattage ATX unit. Unless you are constrained by case space limitations, your wallet goes farther with ATX versus SFX and you'll have a longer warranty with the former and likely better acoustics.
It's worth noting the final sentence from the PC World article:
"Buy a power supply with an efficiency that fits your situation, not the demands of other people who believe you must always maximize on specs."