Like you mention, the leaded to lead-free solder transition was one of them. I agree about 90s PCs quite likely being reliable. But then… 1999 to 2007 we had the capacitor plague: https://en.m.wikipedia.org/wiki/Capacitor_plague
And around 2002 there was the Great Fujitsu Hard Drive Recall: https://www.theregister.com/2002/10/31/fujitsu_faces_lawsuit... If I’m remembering that one, they changed out a flame retardant additive in their chip packaging? Something like that? Those drives were approaching a 100% field failure rate and you could mail them back to Fujitsu and get a cheque for $100.
Fortunately, those beige^black boxes hold no sentimental value whatsoever and will be replaced with something better at a convenient time. Before turning on something more rare, I'd inspect the MB visually for corrosion, bloated caps and test the fans and disk drives externally.