I read the first 3-4, but had the exact same problem with the series I had with most other anthologies of the era: the characters I liked, or storylines I liked, would appear once or twice and then fall out of the storyline. Same issue with the Wildcards series - tons of stories about various entities I couldn’t care less about, but the more interesting, or flawed, or unique ones were almost always one-shot stories.
@Wendell_Wagner also mentioned the series in the linked thread, where I mentioned the exact same issue.
As for your secondary points, well, there’s the oft-repeated issues about creative stories coming from tortured souls. The issues for HP Lovecraft, Robert E Howard, and other authors of earlier generations show it’s far from limited to the authors of the 70s.