Just speaking from experience, but I think the reason people struggle more with katakana as opposed to hiragana is that a large number of the characters look
very similar; the most egregious offenders being ソ (so) vs ン
and シ (shi) vs ツ (tsu).
Hiragana has this, too, with ぬ (nu) and め (me), ね (ne) and わ (wa), る (ru) and ろ (ro), and あ (a) and お (o), but for most of them, the little curled finial is a distinctive enough factor that only the most illegible handwriting would make it so you couldn't really tell the difference once you know which is which.
That's not really the cast with katakana, where the barest difference in angle can not only change the character entirely, but also the word you're writing.