People seem to be caught up on all the things she can do, but fail to realize that she has a ton of negatives in her kit to balance it out.
First of all, her autoattacks are slow. The cast time on her autos is 0.5 seconds, where most ADCs have a cast time of 0.2-0.3 on their autos. She hits like a truck, but her autos leave her vulnerable to skillshots.
Senna may get free AD from picking up mist, but her AD doesn't go up per level. She gets free crit chance, but her crits do reduced damage.
Her Q is dodgeable. It's just like Lucian's Q. The cast isn't instant, and you can get out the way. The zone that damage enemies is actually quite thin. The cooldown is also long and requires her to push the wave (with no mobility) in order to get the cooldown back faster. It also costs a TON of mana, and her mana pool is very small. Despite her long cooldowns, she has mana issues.
Her W is easy to dodge. It's a slow-moving projectile, and even if you get hit, you have a moment before you get rooted to react. It's less consistent than the CC of other marksman like Jhin or Xayah.
Her E has a long cast time and a long cooldown. It shows the enemy exactly where Senna is standing and roughly where her teamamtes are standing. Entering the zone reveals all the people. It isn't a moving Akali shroud.
Her ult has a huge, telegraphed windup, and the damaging zone is small. It can be easily walked out of if she drops it right on top of you. The cooldown is also ferociously long, and the damage is lower than other global skillshot ults like Jinx or Ezreal.
Senna does a lot of things other champions can do, but her tools are less effective than the specialists and on longer cooldowns. She is a generalist, and will get creamed in a straight-up fight if the enemy plays to their strengths and not to hers.