1. Often by the time you actually use it, your ally is either basically fucked anyway or has already rode out most of the CC.
2. The range is small
3. It's a projectile with a travel time, meaning that you often have to cast it BEFORE the target is hit to meaningfully cleanse short CC.
4. The cooldown is pretty large
5. The target has to react to the fact that they were cleansed instantly
6. You have to be sure the target wont just get stunned again
Basically it requires a combination of immaculate prediction, timing, positioning, AND planning to use it optimally. Whenever I do get the perfect cleanse, a lot of the times the person I rescued doesn't even react fast enough to the fact that they were cleansed. It makes me wonder why even bother with this item when pretty much every other support item is significantly easier to use properly while also being more versatile. It's often just better to let my ADC get a QSS or let my midlaner run cleanse.