"Immobilizing effects on Yuumi places You and Me! on a 5-second cooldown."
Grammar issue aside this description is still wrong. Silence puts it on cooldown. Fear puts it on cooldown. Charm puts it on cooldown. The tiny displacement from a Trundle pillar or Anivia wall puts it on cooldown. Literally the only CC that DOESN'T put it on cooldown is slow.
If Yuumi decides to use her passive during a teamfight she is a dead cat. The enemy will catch her out even if they don't mean to. By the time W is ready again either Yuumi or the champion she was going to shield will be dead. Hell in this meta her entire team might be dead.
Fix the description first of all.
Secondly reconsider what behavior the mechanic is supposed to punish. Is it using more than just Q and E? No? It's supposed to punish being caught out? Then why didn't you actually code a caught out condition check like you did with the root mechanic on Jhin's Deadly Flourish? Fix it.