Arcane Comet can be seen flying from the champion using it even if you don't have vision of them, possibly giving away their position.
It appears that Arcane Comet has a delay rather than a travel time; if triggered afar by global abilities, the comet would travel at extreme speed to hit the target just as quickly as it would in close range.
The comet will be disabled on death, but its cooldown will still be able to get reduced by ability hits.
Damaging abilities that apply a slow make it really hard for the enemy to avoid the Arcane Comet.
Hard crowd control abilities almost guarantee to hit the Arcane Comet.