It seems Aurelion Sol became a hot topic of discussion, so i am going to try my best to break down this card being as impartial as possible.
Take your own conclusions if the card looks fair or not at the end.
Lore: Aurelion Sol is one of the Gods who created the Runeterra universe.
Since Aurelion Sol is suposed to be the strongest champion of them all, from a design perspective, is very fitting that his mana cost is set at the highest price possible: 10 mana.
But for a 10 mana cost to see play in any card game, it also requires to have an extremely strong effect.
Here goes a list with a few examples of traits hight costs cards own in other games:
1- Deals high ammounts of direct damage; -> King Crush (Hearthstone)
2- Affetcs the board on the same turn its played -> Shadow Reaper Anduim (Hearthstone)
3- Setup an One Turn Kill combo; -> Antonidas (Hearthstone)
4- Very hard to remove; -> Ultimate Falcon (Yu-gi-Oh!)
5- Generate infinite resource overtime; -> Deathstalker Rexxar (Hearthstone)
6- Snowball overtime (wins by himself if left unchecked); -> Ragnaros(Hearthstone)
7- Denny oponnents resources; -> Jynzo (Yu-gi-Oh!)
8- Protect the player from loss/taking damage/heal; -> Malganis (Hearthstone)
Aurelion Sol is a Boss card, and as such, given he has the highest mana cost price possible for a unit, he is supossed to pack the ultimate bomb effect to win games. He needs to have an even stronger effect then Commander Ledros as he costs more mana and uses a champion slot.
But is Aurelion Sol unfair? To answer that, we need to look at how other Late game decks are doing.
This is a picture of my meme Purssuit deck.
It usually finish games in a single big hit by T9-T12.
This is They Who Endure before nerf.
It usually finish games by T7-T10.
Maokai decks can pull their win condition by T7-T8. That means the oponnent gets 4 more turns as most players can't refill their deck.
It usually finish games by T11-T12.
Ezreal / Karma is an OTK deck. It requires Karma to be enlightned to pull off the combo, followed by Ezreal + spells on the next turn.
It usually finish games by T11-T12.
Anivia needs to be enlightned to proc her egg and level up, plus an additional turn to generate more Anivia copies.
It usually finish games by T11-T12.
Commander Ledros is played on T9 and Atrocity on T10. If Ledros is killed the combo takes longer.
It usually finish games by T10-T12.
Are we seeing a pattern here? Most control decks usually end games around T10-T12. Now lets examinate Aurelion Sol:
Aurelion Sol is played on T10 and if his conditions are met, he levels up after THE END OF THE TURN. So best case possible, the player may only play Celestials cards for free on T11 forward. It usually finish games by T11-T12.
Conclusion: I hope my work here can provide a better insight about Aurelion Sol's design for future reference.
I personally think Aurelion Sol is fair and on par with other control decks that see play on the current meta.
Little extra -> How to play around Aurelion Sol:
Play cheap spells on him to pop his spell shield and follow with a major removal, both with fast speed if possible to prevent counter plays. Another way is to ignore Aurelion Sol completely and focus to kill other units to deny his Level up in the end of the turn. Aurelion Sol by himself is "only" a 10/10 body do nothing with Fury which isn't worth its cost.