Changing a sustained damage mage to a burst mage is a huge change, and inherently a bad one, because league has SO MANY burst mages already. We don't need another. Sustained damage mages are much more rare.
Another problem is, if there is a lot of burst mages, it's hard to find unique things that they offer to pick one over another.
So why would you pick Aurelion?
Honestly, right now there is absolutely no reason.
CC - He offers a very hard to hit stun which doubles as an incredibly unreliable, risky initiation tool with little payoff. His ultimate - while not bad - is still a somewhat long cooldown for mediocre damage, mediocre AoE and very short CC. So there is A LOT of better options for CC (Annie, Syndra, Orianna, Anivia, Cassiopeia, Gragas, Karma, Lissandra, Lux, Neeko, Veigar, Zilean).
Burst - His burst is absolutely garbage. It takes far too long to apply, can very easily be avoided and even if you hit it all, it does pitiful damage. I don't even need to list mages that do a better job, because literally any mage does a better job damage-wise.
AoE - Again, there is much better mages for that. Any poke mage has three times the range and better, more reliable AoE-damage. Ziggs, Xerath, Lux, just to name a few (Lux also has much better CC and burst, so she beats Sol in three aspects).
Roaming - Granted, his roaming is still good, but incredibly reliant on his E now. Before his passive E did a great job to offer him options outside of active E, now you're FORCED to use active E for roaming, because it's all you got.
Also 325 movement speed for a short-ranged roaming mage that is super position reliant and super weak to being dove is an absolute joke.
To be honest, since Sol cannot do anything when jumped on, is very squishy and has no damage anymore, he is actually a terrible roamer, because the enemy jungler doesn't even have to gank your lane anymore, just wait till you move bot/top and kill you in river.
His early game is weak as hell now. His midgame is terrible as well. And his lategame? Well that was supposed to be the big upside of this rework? Well guess what - 0.25 ratio on his stars ENTIRELY nullifies the increased speed of the stars. In fact, at full build it feels like you do considerably less DPS in the first 2-3 seconds, just due to the ratio nerf. So yeah, great job Riot, you removed his sustained damage option while simultaneously reducing his burst.
In short: For ANY situation there is a better pick. Either he needs to be buffed through the roof in damage, health and ratios or you need to considerably increase his utility (stun durations, slows, reliability, etc.). No he isn't unplayably bad - it just feels terrible picking him now because I know for a fact that I could do much better with a different champion in the same situation, no matter the situation.
Maybe before you rework a champion and change his core identity, review if the rework has any actual place in the game or if there is already 20 champs that do the same thing better.