Vel'Koz has no mobility, so Kassadin can close the gap with Riftwalk (R) and interrupt the Life Form Disintegration Ray (R) channel with the Null Sphere (Q) silence to reliably kill him.

Veigar has no mobility, so after level 5 Kassadin can leap over Event Horizon (E) with Riftwalk (R) to engage, break his combo with the Null Sphere (Q) silence, and dominate in the late game.

Lux is a skillshot mage with no mobility, so Void Stone (P) magic resist blunts her poke, and after level 5 Kassadin closes in with Riftwalk (R) to burst her down instantly.

Ziggs has no mobility and deals magic damage, so Kassadin tanks the poke with Void Stone (P) and engages with Riftwalk (R) after level 5 to kill him.

Zed deals physical damage so Void Stone (P) magic resist is useless, and before level 5 he pressures the melee Kassadin with Razor Shuriken (Q) poke and Living Shadow (W) all-ins to snowball.

Talon is a physical carry so he ignores Void Stone (P) defense, and with early melee all-ins and Assassin's Path (E) wall-jump roams he breaks the lane before Kassadin scales.

Pantheon repeatedly beats down the melee Kassadin early with Shield Vault (W) stuns and Comet Spear (Q) poke, and since he deals physical damage Void Stone (P) is useless.

Yasuo blocks Null Sphere (Q) with Wind Wall (W) and neutralizes follow-up harass, and dodges Kassadin's early melee approach with the high mobility of Sweeping Blade (E).

Kennen has no mana, so with Thundering Shuriken (Q) ranged sustained harass he overwhelms the mana-starved Kassadin with poke throughout the early game and takes lane priority.