Many times when you use Warwick's ultimate, the enemy doesn't get suppressed right away (despite the icon indicating they are).
This leaves them open to literally use any spells while under suppression; This only happens within the first .5-.75 seconds or so.
Here are a few examples where the person who gets jumped on is able to cast for a brief moment before actually getting suppressed (Despite his ultimate landing on/already dealing damage to them).
The Cass video is slowed down and is probably the best example.
This feels really terrible when you think you're about to kill someone/lock them down for your team, and all of a sudden they have a "get out of ult free" card just because they use a CC ability the moment they get suppressed.
I've had people use Thresh Lanterns, and any spell you can think of all because Warwick's ultimate isn't suppressing properly.
Riven's W stun and Q knock-back completely interrupt animations (Garen Q jump is the best example), where it places him back on the ground and forces him to re-use the jump animation to cast.
Warwick's ultimate does not have this kind of treatment; the enemy **seems** to be suppressed, but it doesn't interrupt anything they're currently doing, leaving his ultimate feeling very lackluster.