In the almost 30 years of using Mac’s at home and various desktop pc’s in the workplace I don’t think I have ever seen ram fail. Replaced plenty of old school failed disk drives however.
Failing RAM is rarer than it seems from posts online. My theory is that it's so easy to test for that everyone says to do it even if it's unlikely to be your problem. It reminds me of people who needlessly recap (replace capacitors) everything in hopes of it fixing a problem, often not even bothering to test each cap or exhausting other options first. IME dirt/corrosion/oxidation (often solved by cleaning) is a much more prevalent problem than bad caps. After that, solder that needs reflowing is still a more common issue than bad caps.
That being said, I really did have one bad stick of RAM once in my life, and it really does cause strange seemingly random problems.
I think it is less of a concern to the businesses buying these things brand new and more of a concern to the tinkerers who buy/repair/resell/use older models. There's a lot of people who still use ThinkPads made in early 2010s (and earlier). I had RAM module fail on an x270 and replacing it only required opening the laptop (RAM sticks just snap into place). If soldered-on RAM fails, it's game over, or at least full board swap.
Plus, no way to put more RAM/replace RAM with larger module if it's soldered on.
Lucky. Working in repairs I was only seeing the ones that didn’t work, and I’ve seen failures of just about everything. It probably skews my experience.
One time upgrading workstations, 4 of the 20 Corsair kits were sent for RMA. Those aren’t great odds.
I would guess that soldering them to the board reduces the points of failure, the slots can and do fail. However, I’ve also seen soldered components coming off as the cause of failures, but it is usually a part that gets hot combined with a design flaw.
That being said, I really did have one bad stick of RAM once in my life, and it really does cause strange seemingly random problems.