I'm not sure what the current technology is like as I haven't built a PC in a long time, but it used to be the case that efficiency would drop off quite rapidly if you underpower it. e.g. your fancy 600W 95% efficiency PSU would only be 60% efficiency at 200W power draw.
> OEMs often install barely-capable CPU coolers that are noisy (rather than something like a large Noctua cooler that can keep most CPUs cool with barely any noise).
My old AMD Athlon mainboard came with a very rattly and noise cooler for the chipset; I ended up replacing it with an after-market heat-sink that I got for €10. Pretty bad design IMO, but it saved ~€3 on the mainboard price.
A lot of the old Atom CPUs (Atom 330 etc.) had fanless CPUs, but because they were almost always paired with an old inefficient chipset they all came with a little cooler on the chipset. Ended up replacing a few of those, too.
This. I have got an old HP EliteDesk from work. It uses a standard SFF (not sure if it's the right term) PSU, but the motherboard connector is custom. Of course the fan makes a terrible noise. And even if I'm able to find a replacement PSU that would fit in the case (even fanless! - I'm happy with the integrated graphics), I also have to look for an MB connector adaptor.
It's the same with workstations. Although they have plenty of room, but OEMs often install barely-capable CPU coolers that are noisy (rather than something like a large Noctua cooler that can keep most CPUs cool with barely any noise). Or another big annoyance: many OEMs (even reputable brands like Dell) install a PSU that is just enough for the load. If you want to replace the GPU by something that requires a bit more power, you'll end up replacing the PSU as well (assuming that you can get one that is compatible with their custom mainboards).