These "minor" changes really hurt Anivia
The MYMU changes to Anivia did not help her or solidify her identity as the CyroPhoenix. I love her and mained her but these changes made it difficult to even lane against well anyone.
The Changes Q, the base damage and ratio reduced but now the stun gets longer with level only gaining 0.1 seconds/level. This change was fine her Q was always a CC ability rarely used for its damage.
W, Unchanged
E, missile speed is up and casting range was reduced but I haven't noticed any meaningful difference.
R, Her ult is well where there are issues. It now expands to its Normal size pre 6.9, over 3 seconds. After 3 seconds the ult deals 50% more damage and slows more. Meaning her ult slows up to 70% at 16 with it being full size.
Results Her ult before was great way to open up to R->E combo and get off easy damage but now it is much harder because the ult starts off so small. I realize that quick high damage combo has little counterplay but this change makes Anivia's ult well Anticlimactic. It doesn't feel like the ult was compensated for the changes. Sure she gets more slow and damage off but you have to wait 3 seconds before that starts happening. If you are playing against anyone smart enough they will just get out of her ult before it really starts to slow or use a stun or silence to stop her ult. her winrate has been above 50% for a long time and as soon as 6.9 came out it dipped below 50% (Source https://champion.gg/champion/Anivia and http://www.lolking.net/champions/anivia#statistics)
Conclusion Anivia was not conpenstated for the changes to her ult. She has one of the slowest skillshots in the game, Q, and the mana changes offset her mid game power by her visiting the fountain constantly. Maybe her ult could grow larger to be more effective in teamfights or slows could stack and then stun at 5 or 6 stacks. Her waveclear is even better with this more damage on her ult. Riot i just ask you to make her ult more interesting in her kit rather than place and forget.