Toaru Kagaku no Railgun

[Ryllharu]

I don't get why she uses the needles at all. It's really not that effective since they're so small. You have to aim for non-vital organs in a law-enforcement setting (which essentially means upper/lower limbs for guarantee - even then you could hit an artery). She should just use her judo moves that she showed off one time and just drop everyone on their backs (or give them a 3m free-fall.. whatever works). The pins are only good for disabling weapons and firearms.reckon finer control would involve grouping people into channels.

...

We never got to see Uihara's power last season, did we? All I remember was that she can use it after satisfying certain conditions (which were never elaborated on).

That's exactly why she uses the needles. They're light, she can carry a ton of them, and because they are both small and light, she can move them with pinpoint accuracy. The heavier an object, the more variance with her aim at distance (ref: the first episode of s1 when she and Kongou are bickering during the ability examinations, before Kongou sees a real Level 5's power moments later).

Kuroko teleports the needles all over a person's clothes, pinning them to the ground so she can work on the next threat. If your clothing is attached to the ground in eight places (and also very close to your body, gathering the loose fabric), you're not moving easily. Also as you mentioned, it renders guns and other weapons useless with immediate effects.

Uiharu's powers were brought up in the latter half of the first season, when she was holding a bag of taiyaki. I don't remember the episode. Uiharu can keep things at a constant temperature, though she can risk burning herself with something too hot. She probably controls the vibration rate of molecules. The obvious implications is that if she wasn't a Level 1, she would probably be both a pyrokinetic and a cryokinetic at the same time. Mikoto's Level 2 Sisters are a good indication, since they can barely even generate sparks without direct contact, and even then mostly only sense fields, so Uiharu can probably advance past having to touch things.
edit: Index Wiki tells me it was Railgun eps 22.

I assume Kuroko needs to touch things because she needs to use it as a frame of reference before she sends it across 11 dimensions. That way she only has to think/calculate where she wants it, not where it is.