You can't "jump on enemies," you die instantly because her melee form offers no survivability benefits. If you use rappel to catch the enemy there's absolutely nothing you can do to get out. Nidalee, for instance, gets a free leap going in after she hits a spear, so she can pounce away if things get bad. Jayce gets a ton of free armor and MR, and can knock them away with his hammer. He also has a gate if he switches back to ranged form. Elise has... a single target skillshot stun while in ranged form.
If elise wants to utilize spider form, she has to stack survivability. The problem there is she does no damage by doing so. However, stacking AP isn't really worth because she has limited burst damage and very poor AoE.
If you want an assassin, why bring her over Fizz or Ekko? If you want a tank, why bring her over Sej or Malphite? Both offer AoE stuns, are far tankier, and scale better into late game. The only thing she offers is an early stun for ganking.
To make her viable, she needs to be able to fullfill some kind of role. She's a poor assassin, a poor mage, and a poor tank. The MR / armor removal from her ultimate form REALLY hurt her. You can buff her "sustain" ALL YOU WANT but as soon as she gets stunned, she dies.