You're just looking at whats there. Look at whats not there as well.
Best anti adc item around. Doesn't help a whit against mages though.
Best health item in the game. Gives 0 armor and resists.
An ADC's core item. GIves absolutely no attack speed or sustain.
Some call it op. Fact is, its actually gold inefficient unless the opponent has a lot of armor.
And so on and so forth.
Subpar defensive item due to lackluster stats sacrificed for a small amount of aoe dot that doesn't scale well without mpen (which most bruisers and tanks don't build) and no magic resist. Purchase of which can also lead to buyers death through more than one method (baron/dragon agro, position give away through minions and jungle creeps, unnecessary tower shots).
You just listed the obvious, no item is a complete package and does everything, that's why we have six item slots. Sunfire is far from a great stat item but also has massive gamechanging mechanical disadvantages attached as well whereas no other item does something remotely similar.
Let's stop pretending there's any problem with having more control over when you proc the damage, because best case scenerio enemies will still take the same damage and do the same damage as they would facing you if you didn't have a toggle. The item isn't even close to op right now especially compared to the likes of
and
and could stand to use some love anyway. It simply allows for healthier, more intuitive, and normal gameplay.