Allow me to attempt. THE ONLY thing about her I think is too much is how INSANELY easily and reliably she can stack her passive.
Q - Needs SOMETHING to target. Cannot be cast freely. So either needs a WARD, Tower, or champion to target. Is more narrow and precise than it seems, can easily NOT have an Ally and Enemy in the beam at the same time. Has a very long CD that is reduced by autos sure but it also has abysmal ratios and a rather weak base damage. It also has the limitation of only being able to target things in Senna's Auto Range, the effect can go to 1300 but she cant target anything outside of her range.
W - A Slow moving Projectile that is easily dodged or minion blocked and it only does damage to the the target hit. An AoE Root with single target damage that is also DELAYED. The target can react fast enough to prevent it from rooting anyone else besides them if they are smart enough.
E - One of the most powerful abilities in the kit. Stealth mechanics are not new and since she is the center of it you can aim there with AoE and non-targeted attacks. There is a lot to learn and adapt to with this ability but overall its a protection tool that can easily fail. Especially since most people talk about how it makes her faster but that is after a 1 second delay and an animation that is noticeable. So jump and attack as she is activating it and it wont do much.
R - Horrible base damage early, VERY poor AP ratio. 100% FULL AD ratio is good but limited until her stacks are high and also doesn't apply to the shield part. Also ONLY affects Champions so no Drag or Baron steals.
Her Passive and E are the only really NEW-ish aspects of her.
Her passive granting 1 AD per stack is insane with how reliable and quickly a decent player can stack on her.
Her Range and Crit increase are good but limited since it is in increments of 20 stacks.
A good Senna should be focusing on those stacks and getting about 30-50 stacks every 5-8 mins. In a Twenty minute game, as most games are lasting 20-25 mins these days, that is worst case 120-200 stacks. For a range increase of 150-250 from the base of 600 and a passive only crit chance of 90%-100% with the extra being converted into lifesteal.
That is not even mentioning the 120-200 AD bonus, which is the part I think is broken.
The crit would be broken if her crit damage was not SEVERELY gutted. Her Range is nothing we don't ALREADY have in the game in various ways. Kog, Trist, Twitch. The only thing about it that is different is it is PERMANENT. Which would be a problem if not for her HORRIBLE attack Speed. It is REALLY bad even if you go full Attack Speed.
Almost EVERY the end is NEIGH prediction about how insane she was going to be was considered and she got an aspect of her kit GUTTED to make up for the things she was getting:
Slow Attack animation
Severely reduced Crit Damage
Lower Movespeed
Now will there be nerfs for her? Probably. Her stacking is way too reliable at the moment as I have said. But her kit is not that bad. Most of her ratios are REALLY low.
Her W has the highest base damage and ratio between her Q and it and is cheaper at all levels as well as being on a shorter Cooldown, 11 vs 15.
The problem a lot of people are saying is her Ult is Global. Woo. It is EASILY dodgeable and ANNOUNCES itself. Unlike Jinx, Ashe, Ezreal ults which just SHOW UP and do SHIT TONS of damage, or in Ashe's Case a REALLY long stun that either stops your push or gets you killed. The part people don't like is that unlike Ashe and Jinx, it can't be blocked by an ally. The damage part of it acts more like an Ezreal Ult to me but with WAY less damage. It has the same width.
The speed is MUCH faster but it also TELEGRAPHS where it is aimed like a Jinx W. So its like if you took a Jinx W and an Ezreal Ult mashed them together and made them have less damage. (5 rank Jinx W is 210, one rank Senna R is 250, 160% Bonus AD on Jinx W, 100% AD and 40% AP on Senna R. Jinx can fire that thing every 4 seconds)
Overall its not that different from ANYTHING ELSE IN THE GAME. The biggest things are the R Global Shield and the E AoE stealth Mechanic for team. The only aspect that I think would need nerfed at the moment is how reliably and quickly she can stack with almost no risk for doing so.