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.
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.
Plus, no way to put more RAM/replace RAM with larger module if it's soldered on.
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.
I think making it impossible to upgrade is a somewhat bigger problem, at least while the machine is still in-warranty.
Traditionally, RAM has been one of the more-common upgrades to make as needs or budgets change, so soldering it in looks like planned-obsolescence.