Are Dell or HP more reliable than Gigabyte or Asus?

I don't know about "high quality tested parts", at least when it comes to their PSUs. Not to mention there was a long-standing issue with the Dell Optiplex models we use at work, where they would occasionally "forget" that there was a HD installed (the temporary "fix" was to power off the PC & power it back on; permanent fix required a BIOS update, which hasn't been applied to all of them yet).

I would say the testing/quality levels run the same range as BYOP components do: some are good, some are bad, & some are in between. What I will grant you, however, is that they make sure that the components are compatible with each other, & offer drivers for the components. The flipside of that, however, is that the "official" or "acceptable" drivers for the components has to come from the OEM website, rather than the supplier's website (i.e. Gigabyte might have built that motherboard for your Dell system, but the BIOS updates have to be downloaded from Dell's site, not Gigabyte's).

Where they excel on the warranties isn't so much the terms of the warranties -- actual coverage (in terms of what they will consider fixing) & length of warranty period aren't too different than component warranties -- as the fact that, since they sell the system pre-built, it's a comprehensive warranty. A BYOP computer, on the other hand, has multiple warranties to consider; 1 for the GPU, 1 for the CPU, 1 for the motherboard, 1 for the RAM, 1 for the hard drive, etc.

And getting back to the components, & the ability to replace them...they do their best to make it difficult, if not impossible, to replace the components, or at least replace them with anything other than equipment purchased directly from the OEM. Sometimes even finding RAM upgrades can be difficult -- although manufacturers like Crucial have done their best to provide that information, for example. A lot of machines, however, have off-sized PSUs, or use the SFX format, which makes it difficult to both find a replacement/upgrade PSU or find one that gives you the power you want. I remember back in the day how Micro Center's in-house brand of prebuilt PCs (PowerSpec) actually had the components hot-glued to the motherboard, so it was nearly impossible to replace the RAM or expansion cards without taking it into their store for their techs to work on them.