My impression is that adult learners tend to struggle with Kanji even though furigana exists because age is a strong identity factor in Japanese culture and furigana is usually used in age-specific contexts.

So a Kanji that might be accompanied by furigana in a shounen manga (geared to kids) typically won't be accompanied by it in a seinen novel (geared towards young adults). Needless to say, adults don't necessarily want to read doraemon (a kids series) as beginner Japanese learners.