I've built two PCs in the last 5 years and both have had parts that were broken from the factory... There is something to be said about being able to hand off troubleshooting and replacement. I had ASRock pointing fingers at everything else to the point where I had swapped out CPU, PSU, and RAM, until they relented and replaced the motherboard (the replacement which is broken again now 5 years later by the way...)
Do you trust the brand and models of the gear you're buying? Use Newegg for reviews even if you buy from Amazon?
Though if a component dies due to ESD from an imperceptible spark then the manufacturer skimped on ESD testing
- 2x the GPU for about the same price.
- Same power usage while in sleep.
- HDMI CEC with a dongle.
- Wake up from sleep with controller if using official Steam Controller.
- Cool and quiet operation. You can actually beat Steam Machine at noise levels under load.
- Upgradability.
- Standard parts that are easy to service and replace.
What you can't get with a custom PC is:
- Comparable size.
- Comparable support (software/firmware fixes and good warranty/service).
- Cool led strip unless you're into electronics and can make your own from scratch.
Steam Machine can't be upgraded.
Once SM becomes too slow for gaming it will become mostly useless as it lacks necessary ports to be repurposed into a reasonable home server. I always reuse my old PC parts as home server upgrades.
Steam Machine uses a proprietary fan that will be annoying or even impossible to replace 5 years from now.