She may have had alot of nerfs, but non really hut her where it hurts, which is shown by her winrate which has never dropped below 50% since lols beginnings, and for by far most of the time remianed above 53%.
Theres no champ with a higher average winrate over the full lifetime of lol's existence, probably not even one close to it.
The one thing that makes janna the bitch she is is that her Q has any knockup at all on instant cast.
This means that together with her ult, she has two instant aoe abillitys that cancle the majority of all moillity tools in the game, which makes the carry of a wel played janna basically dive-proof.
Not to mention the mobillity she has/grants, the strong slow that means the poor guy that got interuppted mid gapcloser won't get anywhere, usually leaving him easy pray to the carry, the huge amount of effective hp that her ult+shield+sup-items give to keep the carry alive should for whatever reason an enemy actually get to him. and the free ad for the carry....
What janna needs is a sion treatment, let the Q only apply the knock-up if it atleast channled X sec(like 0.5), else it will apply a brief but powerfull slow.
This should free up quite some power to increase the reward for channled Qs aswell as giving allies an indicator, aswell as maybe upping her basehealth a bit to compensate for the lower selfpeal.
And no one can mess up like Alistar. A bad allistar is litterally 4vs6. One failed monsoon can be worse than a failed headbut, but a bad allistar will fail MANY headbutts, it's a far more spamable screw up.