I posted this in another thread not realizing it was the wrong thread, but I feel like this needs to be discussed because these weaknesses are so massive, so here are my thoughts.
Okay so I've been messing around in custom games and here is my initial thoughts about this.
Taliyahs Q ruins the ground for way too long, I can understand what you are going for her, but lets say you were clearing jungle camps (She's not a very good jungler, but still) by time you full clear and go back to the first, the areas you used Q are still useless, this is a fairly hard to use ability, and it's so restricted, I just think this bad, why put all this effort into this when you can just play Malzahar and summon 4 Voidlings on a Target and the point and click watch them melt. Or play Brand and Pentakill clustered enemies with your damage alone.
Taliyahs ultimate does no damage, and doesn't seem to displace, although I've yet to use it on a champion, it doesn't displace minions, however it does create terrain, this is a bad thing because Taliyah has no mobility to travel over the terrain, meaning she has a high risk of screwing herself pretty badly, she's very squishy so flanking with this could just lead to her death and splitting an enemy could just allow more mobile Adcs like Lucian to destroy her since he can choose which side of the wall he wants to be on at any time.
Her E is slow but it's okay and her W is okay, these abilities don't have flaws on the level of Q or R but they don't seem that great either, add in the fact that her passive has no combat use and it all adds up to this champion seeming unreasonably under powered.
I also haven't looked at the numbers of her base stats but she feels squishier than most mages, maybe I'm wrong about this one but I definitely feel like this champion should be at the very least equally durable to the average mage, but definitely not squishier.