The mistake is treating kitten heels as the safe, grown up option and then wondering why the whole outfit looks a bit dated and fussy. They are not a smaller, more sensible stiletto. They sit at an odd little height, usually somewhere between three and five centimetres, and that height changes how a trouser hem sits and how a bare ankle looks far more than people expect from something so modest.
Keep them away from anything already fussy up top, a heavily ruffled blouse or a very girlish dress, because the shoe has a slightly prim quality on its own and two prim things together tip into costume. They also don't do well with wide, flowing trouser legs that puddle over the foot, since the whole point of the low, slim heel is a bit of ankle and instep on show, and a dragging hem just hides the shoe you paid for. Where they properly struggle is distance and speed. Anyone expecting to do a long day on your feet, a lot of standing, or any kind of hill or cobbled street, will find the narrow heel and short block height give you less stability than a flat but still all the discomfort of a heel, without the obvious grip of a slightly higher, broader heel to balance your weight over. They're also not forgiving if you have wide feet or a high instep, since the style leans towards a narrow, pointed last, so a shoe that looks fine on the shelf can pinch across the widest part of your foot within the hour. Before buying, check where the vamp sits, whether it's cut low enough to show some foot or whether it comes right up and makes your foot look shorter, check the heel itself is sturdy rather than a thin spike pretending to be dainty, and check the insole, because at this height there's often no padding at all between you and the leather. If you've got none in your collection, start with a plain pointed court in a neutral colour, nothing with a buckle or a bow, since that version goes under trousers, under a midi skirt, under a dress, and doesn't fight with anything else you're wearing that day. A neutral also means you're not trying to match a specific shade of beige or tan across different outfits, which with heels this low really shows if it's slightly off.
They are a lovely, quiet alternative to a flat for anyone who wants a bit of height without the commitment of a proper heel, but they were never built for a long day on hard pavement, and no amount of good styling changes that.