I have always used a UPS power backup. Have had almost no hardware failures in over 15 years, except one portable hard drive, a few GPU's and a few CD-DVD/RW drives. My motherboards, HDDs, and power supplies have always seemed to outlive their usefulness.
So power and heat (plus EMI or solar flares - you need to build a faraday cage for that
https://en.wikipedia.org/wiki/Faraday_cage) are probably the biggest threats. Always use a top quality power supply, good case fans and cooling setup. I really believe a good UPS keeps the power input cleaner to the PC power supply. No spikes or surges, etc. should benefit in longer component life.
Of course if you buy circuit boards with crappy capacitors, you get what you pay for. I've replaced boards for friends that have expired due to leaking, bulging, capacitors. Always buy quality components!