One element that gives strength to Malz is his ability to waveclear, which is highly overlooked. Many games I've played and won/lost, the losing team lacked that focus. With Malz, it's purely incidental. This alone boosts his winrate beyond his core balance.
As to his balance. His voidlings are overtuned on the power/spammability side. Getting multiple out fast should be about when you do it, not necessarily always being capable of doing it. When they do come out, they do a hefty chunk of potential damage, which when focused with a proper Malefic Visions, is very difficult to counteract those multiple targets without disengaging from Malz.
These two things could use tweaks, not necessarily nerfs and I think he'd be manageable in many regards.
Everyone has stated the concern with an uncleansable ulti and while that's a concern, the impact his ultimate is supposed to have is just that. The DPS portion of it is equally strong though and that's the problem. It's not a balance of CC OR damage, but instead CC AND damage.