what makes the build objectively bad? It's not his scaling surely, because he has some of the highest AD scaling even among pure AD champions. I hope you're not thinking it's his utility.
AD malzahar is a low mobility AD caster. He deals mixed damage has 2 forms of CC, % health damage, and (unlike just about every other AD caster, excluding jayce's hammer W passive) has mana regen built into his kit.
Now, numerically, his base damages are the highest among just about any AD caster or marksmen (300, 40%, 320, 550).
Then AD scaling wise, at 45% CDR you can get 3 voidlings out for about 4-5 seconds.
Assuming you'd have a voidling in each stage of their growth, you'd have 100+150+150+100(malzahar's autos) = 500% AD scaling (assuming his attack doesn't crit), each attacking on average once per second. So what other champion can get off 5 auto attacks in 1 second?
This is going to sound crazy to you, but i'm actually an AD mazlahar main. He is by no means weak. Ironically riot ended up having to nerf him. A while back riot removed the 20% bonus damage to minions (and pets) on
. And removed 20% move speed to nearby minions(and pets) from
. So explain to me how those nerfs majorly impacted any other champion other than malzahar. Tibbers didn't care, Zyra didn't care, Azir wasn't out, It was pre Morde rework, and riot forgot yorick mori.
So take that 500% damage and increases it by 20% if you're support bought a banner (600%), and captain boots increased reliability and chase potential of the voidlings.