The amount of mana that she goes through for her combo now that she has to use her wall to reliably use her E (unless the enemy couldn't dodge her Q) is her real counterplay to her ultimate. Not to mention that you can still stop her ultimate by hitting her with CC which puts her ultimate on CD and basically means she has no real damage until that comes back around.
Also while she has amazing wave clear that is probably the best in the game and she can stall out games pretty well, that is only when she hits level 6 because her pre-6 wave clear is down right terrible. So take advantage of that and constantly push her into turret pre 6, because she is so bad at last hitting under tower and I'd say maybe one of the worst pre 6. Then when you think about Mid/late game, sure her wave clear is insane but it costs her like 3 or 4 times as much mana to really clear the waves when compared to other strong wave clear champions.
Like
has to have her ultimate out for X amount of seconds while also using her Q (or just use only her ultimate and lose more mana) which could easily reach 300-400 mana, especially if the enemy team has Baron which would run her mana down even more.
Compare that to some one like
who just uses his E and can clear an entire minion wave with only losing like what 110 mana? I'm to lazy to check his E's mana cost but its probably around 100-120 or so.
Personally there is a lot of counters to her and Anivia has a lot of weaknesses which I'd still argue that she has one of the most amount of weaknesses in the game. As an Anivia main, I always look forward to fighting her if one actually gets picked into me Mid lane because I haven't lost a lane against one because I know how easy she is to beat in lane.
I feel she is perfectly fine the way she is. She doesn't need buffs at all and she isn't to strong to need nerfs. She only "feels" like a really strong mid laner because most people who play her now are more experienced Anivia players.