It has lots of multi-core performance
I agree with your post, with the exception of your statement about the M1 multi-core performance.
At a minimum, I would say they M1 has excellent multi-core performance for an entry or even mid-level Mac, but in context to being a Pro machine, I wouldn't say the M1 has "lots of" multi-core performance.
Just for comparison, the now four year old 2017 iMac Pro destroys the M1 in terms of multi-core performance. Even the base model iMac Pro is significantly higher than the M1.
Not saying the M1 doesn't have impressive multi-core performance, actually I am saying it has excellent performance, but in context to the OP question about why the M1 is "not good enough" all of a sudden, multi-core performance is a major factor when comparing the M1 to much older Intel Macs.
I’ve never heard anything bad about the M1.
As impressive as the M1 is, there is some downsides. The lack of multi-monitor support is one big example, and a total deal-killer for some.
Still, the M1 is impressive for the price point. The M1 Mac Mini has the fastest single-core performance of any Mac in the history of Macs, but it is also the cheapest Mac in the line up. Might even be the cheapest Mac ever when inflation is taken in consideration.