The only way I can make sense of this sentiment is that people feel like CSS is something to hack with, it is a toy, for hackers to play with, not a tool for professionals who spend years mastering the skill. I wonder why this is.
It lives in this weird in between land where it's not really a programming language but it's complex enough to be hard to use by designers who are not really into programming.
I agree with you that mastering frontend requires a lot of time and practice, especially because it's a constantly moving target with new features, new tools and browsers that are constantly evolving.
I personally like it, I love the flexibility and I love that it's constantly changing and I have new tools I can use. It's also funny how there seems to be always something missing.
Perhaps there is a lack of understanding of the interaction between a web-developer and a web-designer which creates this unique negative perception of CSS as a tool. A back end developer might simply not understand it.
You said it yourself “once you get into the coding for a reasonably big frontend”. That’s a specific task and the web is not made of only big projects.
CSS is vast but also allows you to ease into it. You don’t have to master it from day one in order to use it.
And that’s the beauty of it imo. I personally enjoy working with CSS and one of the reasons why I like it it’s because after 10 years and hundreds of sites coded I still learn new things about it.