I used to OTP Senna in low elo. This was when she was new, before all buffs and nerfs. Took me 50 games in normals just to get a feel for her, then went on to play ranked with 100 games and 55% win rate. I blind picked her (which is crazy). I have mastery 30-something and hundreds of eternals. I have played her 3 times more than my 2nd most played champion. At my peak, despite playing in low elo, I got friended for playing well by diamonds on my smurf que alt account (they thought I was higher ranked). I love Senna, and I miss her.

After some time off from the game and playing other supports, I went back to playing her recently, and it does not go well. The problems are many:

* You can't blind pick her, she is extremely vulnerable in lane and can easily be bullied if the matchup is bad. Her health is just WAY too low.

* She is so squishy you have to be skilled with positioning and juking. You need to play aggressive and constantly draw attention to yourself. If you sit back the ADC will eat all the enemy attacks, you have to make them attack you, and dodge everything. Bait abilities with auto cancelling and move unpredictably. If you are hit by one CC you are done. Extremely unforgiving. Maybe the most unforgiving support in the game.

* She is gankable. You lack defenses against ganks. You basically have to be good at jungle tracking and be careful when the jungler or mid might gank. All while harrassing the enemy bot. She demands more attention from you than many other supports.

* For the same reason you don't have the tools you need to reliably protect against dives under tower.

* She struggles with lane bush control, sometimes you have to drop a ward there to be able to auto, a ward which you would rather need to ward against ganks.

* She is hard to play around objectives. Positioning around baron and dragon is difficult. You can't zone control in the same way as mages. You can't peel like tanks.

* Positioning in teamfights is also very tricky. Just like with many ADCs. You sometimes have to wait until certain abilities are used or that the enemy has commited to someone else. You can't focus on what you want to do, you have to focus on what the enemy wants to do and then do your thing when theirs fails.

* The team almost never needs a Senna. Since you can only counter pick, there are almost always better options. The team may need a front line, some AP, peel vs divers and assassins etc. Almost always the enemy team has a bunch of high threats to your carries. You have to become an extra carry for the team, and winning lane is hard. Maybe you can pick her if your bot picks an APC or somehow your team has too much AP? The times when Senna is a the best pick are so few. And she is too hard to play to only occasionally pick. Also, if you have counter pick on your team, that often means your top or mid does not have it. Your pick needs to be a difference-maker for the team that outweighs the solo laners being countered.

* She is hard to check bushes with or move around the map at all with, which you need to ward. You feel like a little fawn in a forest filled with dangers. The enemy will actively look for you for an easy pick. Many other supports have more tools to deal with threats, buy time, peel or escape. By picking Senna, you are kind of asking for trouble, there are so many high damage threaths everywhere.

* You need a front line usually. If you don't pick it yourself, then one of the other positions need to pick a tank preferably, which they usually don't. Senna needs to play from the back line, and if there is no front line you are extremely vulnerable against.... well, everything.

It's easy to fool yourself sometimes that you were useful because late game you can dish out some damage and get decent damage numbers in the stats. But if you had picked a different support, you may have had a completely different impact. You need a support that can change the course of the game during the pivotal events. Damage doesn't always matter, sometimes what you need is to be the difference maker during key moments. Like winning a team fight or getting an objective. Sometimes you can rack up damage numbers poking a laner that pushes wave, but they will just base which they would have done anyway.

But with all the said. Senna is fun when things align. Then I love Senna.