The law is having trouble catching up on this front, especially in a world where any YouTuber with an audience could potentially run undisclosed ads. Regulating people posting videos out of their basement sounds like a nightmare.
That said, I do my best to never support content creators that don't disclose ads or hide their disclosures in hashtags or below the "Show more" button in a YouTube description. That's not a real disclosure. You know it and I know it. That's why you put it there in the first place- because no one will see it.