To share some own experience: I also upgraded lots of PCs (mainly at work) from HDD to SSD to get some more life out of them, and this usually went well.
In my experience (from about 25 years of building and repairing PCs) PCs can fail after some period of no or minimum use due to several reasons.
Those include dust and dirt buildup that causes heat issues and/or attracts humidity with leakage currents, failing powersupplies that run unstable, and especially in systems that ran 24/7 for some time, minor cracks in the PCB itself or dry electrolytic caps. After getting cold, on power-on there would be some issues...
But this should not amount to about 30% failure rate.
Some good ideas when upgrading those old systems:
- Clean for dust and dirt with compressed air
- put new thermal paste between CPU and heatsink
- check all fans for smooth operation
- check all internal cables for tight fit- sometimes it is worth unplugging and replugging them a few times to get rid of some corrosion/oxidation
- same goes sometimes for RAM modules- in earlier times where not all contacts were gold plated, the SIMM or PS/2 modules were cleaned with a eraser/rubber- blue side, if needed...
Other story: A year ago, I built myself a new PC from scratch and left my old box in the corner (minus the HDD/SSDs). Two weeks ago I wanted to repurpose that machine as a Unix workstation for some fun stuff related to photography. So took out that system, cleaned it a bit, hooked the original SSD (120 GB Samsung before they were called EVO...) and tried to power on. Nothing. Some beeps pointed to memory issues.
Did the whole stuff of swapping out memory, removing everything that would not be needed for initial powerup- nada.
Put in a CPU from another system with the same socket 1155, and on it went like a charm.
Conclusion: In about the roughly 10 months the system stood aside something happened with the CPU- a genuine Xean 1230 v3 that worked about 7 years or so without a problem...
Mainboard was/is Gigabyte, one of the first models of their "Ultra Durable Series" with a H77 chipset- solid stuff, this combo worked long hours without any hiccups...