You guys misunderstand. When they said multi-hit spells they meant spells that tick multiple times on the same enemy such as Malzahar's Malefic Visions or Mordekaiser's Children of the Grave.
The slow is weaker on those spells because they effectively extend the duration of the slow. Malefic Visions lasts 4 seconds so with a 1 second slow per tick you end up with a 5 second slow. If that was a 40% slow it would be incredibly powerful.
They changed it so that spells flagged as "Area of Effect" now get a 40% slow, but the spells flagged as "Damage over Time" still get a 15% slow.
Bouncing spells like Kat's Q are flagged as "Area of Effect", the same as her W.
Some spells like Cass's Q are flagged as both AoE and DoT since they debuff multiple enemies with a DoT. That means that it has reduced effect from spell vamp and only gets the 15% slow from rylai's.