Honestly, Akali has a better chance of surviving mid combat than Diana...

If you look at fighter/assassin who dived right into the fray, they all have better defenses than Diana.

Yasuo has his windwall and a shield that can be reset with this ult. Plus his aoe stun. Xin has his heal/knockup/knockback/ult shield from range. Jax has his E that blocks auto attacks and a massive armor/MR steroid from his ult. Plus his aoe stun. Akail can hide in her shroud for days plus she has some heal.

My point is, Diana is kinda fragile considering she dives in and pull enemies towards her when compared to other champs.