Janna is good vs poke and engage champions
What she will struggle vs are blitz (can't stop the engage cause you go to him)
Burst (Mage supports like Annie and brand)
And denial champions like Morgana for spell shield her CC and deal more damage than her
I would argue poppy is also good vs her as a support
However this is in lane, late game a good Janna can overcome her weaknesses by counter building for the occasion
Please also note Janna damage comes from buffing her adc ad, and adc are a bit out of control right now, riot hit here here with the E nerfs