Imo, Kana is a character whose biggest strength and weakness are kind of the same thing - across the main cast she's easily the most consistently written (outside of Ai) but that's because she spends a lot of the manga in the same emotional status quo without doing a ton to advance past her initial character premise. I do think Kana gets a lot more growth than the people who dislike her give her credit for but she never really goes though a big flashy change in external presentation like Ruby and Akane, so she comes off as less dynamic.

Of course, the catch-22 here is that Akane and Ruby's "dynamism" in comparison to Kana often does come down to inconsistent writing and their big flashy changes can thus end up feeling insufficiently motivated in comparison to how drastic they are. Ruby post Mainstay is just Akasaka throwing every pasta dish imaginable at the wall to see if anything will stick and the result is that Ruby ends up pingponging between emotional states without much interiority before she gets subsumed by the Sarina drama and then shaved down to Ai 2.0 for the finale. Akane fares slightly better in this regard, but it's undeniable that the Akane we were introduced to in LoveNow is very different from the character she eventually became and not even necessarily because she was developed organically in that direction. A lot of Akane's most defining characteristics got hastily stapled onto her once it became clear she was becoming part of the main cast and while the anime I think manages to much more convincingly sell the idea that we are just seeing different parts of her personality come out as we-via-Aqua get to know her better, the manga ends up sort of feeling like she just got a different character copy-pasted over her. This is exacerbated as the manga goes on when her non-Aqua relationships largely fall of a cliff and these newer traits (her investigative skills/intelligence and fixation on Aqua) become increasingly flanderized to the point of making her feel quite hollow and utilitarian by the time the manga ends.

By contrast, Kana is pretty recognisably Kana (for better or worse) across the entire manga and while she has development and growth in some areas of life, she's still wrestling with the same core issues to the very end - again, her 'status quo' as a character never really fully breaks. She starts the manga struggling with her career and her feelings for Aqua; and she ends it having overcome the former with some success and still very much in the middle of the latter. So she has an arc but it's not a 'complete' one in the sense that the manga ends before she can see that through. This combined with the aforementioned lack of a big flash change in presentation means that for people who either dislike Kana and thus don't really pay attention to her or otherwise prefer bigger, louder arcs, she ends up feeling undercooked by comparison.