Q:

Are some motherboard brands, per se, of lower quality?

I have heard in different occasions that some motherboard brands (such as PC Chips and ASRock) are cheap, in both senses: less expensive and of lower quality, because the manufacturers use components that don't reach certain standars and/or have been discarded from assembly lines with higher standars (like ASrock that would be a cheap line from Asus).
Are these affirmations true or complete nonsense?

A:

Per se lower quality?  No.
There are lines which produce very cheap motherboards. I suspect (-with no hard evidence to back that up!-) that most use cheaper components.
However things get quite muddled at the cheaper end. E.g. consider the Cyrix 200+ CPU. Sold very cheap compared to the pentium-1. Often found in desktops with the cheapest possible power supplies, the cheapest possible motherboards, the cheapest possible graphics cards... and often in an unstable system.
Unsurprisingly it had a very bad name, yet one ran without problems for years in an old Asus boards with good hardware.
The point of that anecdote is to point out that a piece of cheap hardware can be worse quality, but it does not have to be. And that mouth to mouth experience in this case is often wrong.
Having said that: Your motherboard is the backplane of all other components. Both on add-in cards or integrated. Getting a good motherboard is worth it. So do read reviews of which board you are considering, but do so with a critical eye.

