So let me preface this post by mentioning that I am a Challenger level Aurelion onetrick so my opinion is very obviously biased.
It is also important to note that before this split Aurelion was incredibly strong and despite what most people would have you think was in need of a slight nerf.
Now the main issues with the champion are two-fold. The main one and point of discussion is the removal of Comet's cooldown refund on ability damage. Aurelion had incredible synergy with the rune because his Q counted as both a DoT and a single target spell. As such Comets cooldown was rapidly reduced and the damage numbers would reach incredibly high amounts sometimes being 15-20% of the champion's total damage.
DFT simply cannot compare to to Comet in terms of damage. Even before the hotfix nerfs when it was overpowered.
Even ignoring the huge damage disparity, losing comet pretty much removed Asol's poke in lane and a very important form of skill expression that made him viable in high elo midlane. Making his early game much worse than it already was.
This serves to push him even further away from mid and making his only Viable in high elo role Botlane APC. Which is a role he was already heavily pushed into ever since his hitbox was nerfed to be the same size as tanks like Skarner and Ornn.
The main problem with both of these changes is that they are indirect heavy nerfs that effect both the champion's versatility and direct powerlevel. Pushing an already very one dimensional and simple champion to an even simpler play pattern.
I for one dont want direct number buffs however the change I would like to see is changing how his Q is treated for runes such as electrocute and Conqueror. Since it was not allowed to proc them because the comet interaction would be even stronger.
And if im being completely honest and hopelessly optimistic the hitbox change should be reverted so he can properly lane against skillshots again.
OpGG for anyone curious: https://op.gg/lol/summoners/euw/Backslicer-KekW?queue_type=SOLORANKED