tl;dr: the issue isn’t the @import rule itself, but that files under 1kb often end up the same size or even bigger when gzipped, so you get no compression benefits.

The experience shows that atomic css files is not optimal.

If the files I was importing were larger, it might make sense. As tiny, modular files? Not so much!
The complete library concatenated and gzipped is less than a single HTTP request. It’s just over 25-percent of the transfer size of sending modular gzipped files instead.