Hell, from a balance standpoint, I'd argue that uniques in of themselves aren't even the biggest concern!
I think that's Malek's problem.
Most unique items have drawbacks : Necroarmor gives great ms and 6 armor, but drains your life, Lava is annoying to repair, shielded is very specific, cybernetic is cursed, and medical... sucks.
Malek ? Shines on all points : good protections, good ms, and regenerates quickly, for free.
Even enviroboots, which are 5% slower than A-modded cerberi boots, are not interresting on angel of grind.
And it's the same for weapons : Trigun or BFG10K have drawbacks, one with its reload speed and the other with friendly fire.
Malek and its boots pair are just too good.
I'm already happy enough when I find a necro, phaseshift, or nuclear plasma rifle !
Should rarity be a counterbalance for power ? Well, this is a roguelike, so of course, average items (vanilla weapons and armors, etc.) have to be common, while some better ones need to be rare.
But I hate items witch completely change the game, like Malek's ones, or that stupid DS.
Currently, they usually occur late enough, so it doesn't change much the game, since the hard part is Phobos. But I hope we'll fear hell again in a few versions, and the problem of OP stuff making it a cake will come back again.