Janna has decent skill expression in her kit.

She can interrupt dashes and stuff like that with her ultimate and q spell. She can make plays with her ult (imagine flashing behind someone) or simply catch somebody off guard with a max charged tornado.

BUT

She doesn't need to do these things. Inexperienced jannas can do great on her by just spamming e with censer and coin while never running out of mana cause of coin.

You don't even need to interrupt a khazix or rengar jumping at your adc. You can straight up make your adc be able to face tank his burst with long shield duration and free stats plus heals.

That's just boring gameplay and also her favorite keystone (forgot the name) gives even more free stats in form of resistances. GREAT.