"Rape Kits", or rather, the standardization of process for collecting evidence after a sexual assault, are surprisingly good at collecting usable (convictable) evidence after a sexual assault.

The biggest problem with them is that roughly half of them go untested. When they are tested, however, they're actually incredibly good at the successful prosecution and conviction of sexual assault cases.

When the rape was reported soon enough for evidence to be collectable; and when it was a form of rape that leaves a distinctive mark relative to consensual sex. I do not know how prototypical these both of these conditions are; but at most, you would reduce the "usual" qualified to "non-negligible", as there certainly are cases that do not meat the criterion to have a usable rape kit.

(Unrelated note, in writing this comment, I tried looking for relevant statistics; but found I have no idea what to even search for. Anyone have anything?)