Assuming quality components, it’s fairly rare for a bum part. Just annoying to try to troubleshoot which part is a dud and then have the RMA it for a replacement (with the extra shipping time, etc). That’s probably the primary advantage pre- or custom builts have over DYI jobs – already tested and a single warranty.
DOA rather than assembly error, I can’t recall anything other than one or two sticks of RAM being bad during the initial build out of all the systems I have build over the past 15 years. Maybe 30-40 systems total. I’ve had a bunch of motherboard, ram, and power supply failures in use, one cpu ever, and one case of a short in the fan cable causing the system to not boot. Power supply failures mostly early on, virtually none since I started buying high quality 80+ bronze or better supplies. I usually check jonnyguru.com for that.
So, the specs are looking like this:
CPU: Intel - Core i5-9600K 3.7 GHz 6-Core
MB: ASRock - Z390 Phantom Gaming 4 ATX LGA1151
RAM: Corsair - Vengeance LPX 16 GB (2 x 8 GB) DDR4-3000
SSD: ADATA - Ultimate SU800 1 TB 2.5"
Thermal paste: Thermal Grizzly - Kryonaut 1g
CPU Cooler: be quiet! - Dark Rock 2 57.9 CFM Fluid Dynamic Bearing
Case: be quiet! - SILENT BASE 601
PSU: EVGA - SuperNOVA G1+ 750 W 80+
I’ll put my old GTX 970 until GPUs come down in price.
Jophiel’s note about NVMe SSDs not being worth it is well taken. I realized that if someone wants wide bus, low latency solid state memory, they already have RAM and VRAM in their system so unless they can’t or don’t want to use it for some reason, NVMe makes little difference unless you have a very specific use for it.
I like https://www.reddit.com/r/PCMasterRace/wiki/builds and https://pcpartpicker.com/guide/ for building your own PC.