She is easy to pick up, but hard to master and be really good with her to reach those high elo. Decision making is what is the hardest. I'll explain.
As you said, her kit doesn't seem hard to use and it's straightforward and that's why she's easy to pick up and start off if you're looking to start as support, and that's why she's my main and only support.
But as you get to higher elo, you see uses of her kit that don't seem too obvious at first.
Her passive might feel lack luster, but it can do a lot of things. It can help your ADC get back in lane faster by staying close to them and them receiving an MS boost. Sometimes, those 3-4 seconds they get in lane faster might make a difference in xp and gold. Just think that Riot BUFFED the cloud drake to match the MS bonus that Janna grants her allies. Also, during a team fight, it really helps by having correct positioning to help allies kite around the fight.
Her Q seems simple, but in the hands of a proficient / skilled player, it can work wonders. Lee Sin is the most popular jungler right now, and Janna can shut him down with proper and timed uses of her Q. Having quick reflexes can be key. She can interrupt so many dashes-dives,but that's not something that everyone can do. Also, fully understanding the speed of your Q can make a big difference in the amount of time you spend charging it up. If a target is slowed or CCed, you can opt into charging it more and hita bigger knock up, but that requires a lot of experience.
Her W is mostly decision making. Should you use it or not. Not hard to decide really, so I'll skip to the next ability.
Her E is also easy to use. In lane, shield whoever needs shielding the most, or your ADC in general if he utilizes poke, to increase the damage output. In teamfights, you might need to think a bit more between shielding your diver, or your ADC because you need to choose what's more important; survival of the tank (if they even need it), or survival + dmg increase from your carry. All these need on-the-spot thinking.
And of course her R. That's the most diffcult ability to use, and one of the hardest to use correctly in the game. To use it properly, you need to watch all 10 players during the teamfight and see if it's worth using and when. Sometimes, ulting to save your teammate from dying (be that through denying a dash or straight up healing) might not be a good decision, because you might be also saving enemies from death, which would be advantageous for your own team. Saving your Lee Sin, but also saving the enemy Syndra isn't a good trade off. So much depend on correct use of this ability, that it might just straight up win you the fight, because you completely turned a losing fight into a win by healing the entire team and interrupting / denying the enemies, but it can also lose you the teamfight, because the repositioning of the enemies might actually benefit them.
Do you know what is a straightforward and easy support to pick up?
-Sona. Just spam abilities and hit your ulti. And maybe some decision making on which skill you use your empowered auto with.
-Soraka. Just heal whoever needs healing, ulti when someone needs more healing and hit your Q and try to silence enemies.
-Nami. Just hit your Q and R and then E your ADC and W your allies.
-Braum. Get in front of someone, raise that big shield and get off a good ult.
Those supports are easy to utilize. Janna's R is really hard to utilize correctly and you can easily solo win or lose a teamfight by using it.
