Very different situations, Ekko has survivability outside of his w, q slows, passive slows + ms boost to run away, e double jump, ult teleport. Veigar on the other hand has only his stun.
Not mention Ekko's stuns everything in its fairly large radius for the same duration as a rank 4 veigar stun at rank 1 + gives ekko a big ass shield even if he misses the stun. Veigar's stun on the other hand only affects those who run into its walls.
Ekko does not require his stun to land his damage, veigar does.
Just place it behind the enemy and they either walk into it or face your Q and R, which if you know how to cs with Q, will chunk them.
Thats the thing, as a squishy front loaded bust mage you dont want to chunk people you want to kill them, if you've blown all your cd's and the enemy is still alive you have nothing to stop them turning around and murdering you.
Hypothetical 1: Veigar catches jinx alone, jinx has 0 defence items and veigar is a burst mage. Veigar throws down his cage. Jinx turns around and starts autoing veigars face, W is not going to land so you throw out r q. It "chunks" jinx. Jinx proceeds to kill veigar and live another day.
Hypothetical 2: Leblanc catches jinx alone, jinx has 0 defence items and leblanc is a burst mage. Leblanc full combo's and jinx dies.
Hypothetical 3: Annie catches jinx alone, jinx has 0 defence items and annie is a burst mage. Annie full combo's and jinx dies.
It is not up to veigar if his combo lands, it is up to the enemy.