Double ADCs are notoriously bad come late game, there's near 0 survivability, they don't have items like Zhonyas or cc or mobility like most tops mids and junglers have (except ezreal but he pays for it with damage output), so it's in a sense easier to get aced. All the dangers of losing a person late game are made easier by the fact that now you have 2 total squishballs on your team.
As for top lane matchups. I haven't laned against it but nasus sticks it where the sun don't shine since wither literally cripples Kalista in more ways than one. That said that comes with the problem of playing nasus, a la: I'm a puppy til 30 minutes, plz don't hurt me.
I imagine with some shrewd usage of her stun and Q irelia could do fine...
AP Malphite perhaps? But that's a bit oddball....