Slow does crap to reduce an opponents saves, and doesn't help much against a brute monster that can just Power attack for 10 and still clobber someone with a single attack.
If a bad guy hits you on a 2+, giving him 2 opportunities to roll a 1 is such a minor increase in, its barely worth noting. You'd be better off casting Blur or Displacement or something that gives a greater miss chance or hitting your foe with something like a Ray of Enfeeblement or Ray of Exhaustion that doesn't allow a save than Unluck.
Unluck is similar to any other save or suck spell...it's better than most save or die spells, because it targets an easier save. But it can affect a lot more creatures types than many of those spells, and it's effective on things that slow can't do much to stop.
No its not. Its worse. List me some examples of things that Unluck would be better than Slow? I mentioned that Slow isn't that effective vs casters, but neither is Unluck. Unluck doesn't stop the wizard from Fireballing you, or Baleful Polymorphing you into a fluffy bunny, or casting Hold Person so his minions can CDG you, or Glitterdusting you so you can't target him. If you have evidence to refute the catagorical analysis I did above, please, post it rather than making broad, unsupported generalizations.
It's like forcing a monster to re-roll every attack/save. I played a fatespinner and that ability turned out to be VERY useful.
I can't think of many PCs that would prefer having to re-roll all their attacks/saves, versus being slowed.
Unlike Fatespinner, though, Unluck relies on a foe first FAILING their save, AND takes an action. Fatespinner requires neither. IF you are gonna spend a whole action and rely on your foe to fail their save, USE SOMETHING ELSE. Seriously, Glitterdust, Slow, Deep Slumber, Phantasmal Killer, Baleful Polymorph, ANYTHING. It is NOT good for setting up 1-2 punches between 2 casters, as I stated above, because if they probably would have failed their save vs the Unluck, they would have failed their save vs something BETTER like Glitterdust or Slow or Iron Bands or Stinking Cloud or Hold Person or anything. And even if they fail their save vs Unluck, that doesn't mean they'll fail their save vs the followup spell, whatever it is. If Unluck doesn't disable a foe on its own, and doesn't garuntee that my next spell will work, then it sounds to me like a waste of an action.
I understand that it works on a large range of foe, so does Slow, so does Glitterdust, both of which give GAME ENDING status debuffs. Seriously, Glitterdust makes you blind, which is essentially the same as making everyone else invisible. That gives everyone 50% miss chance. In order for Unluck to give a 50% miss chance, the attacker has to be able to hit on a 11+ on their attack roll. Most big bruiser foes that you would target with this spell are gonna hit on a 2+. If I was a melee character intent on taking down a Hill Giant, for example, I'd rather it Blinded or Slowed than Unlucked, because it can still full attack me while Unlucked (can't while Slowed) and if it did full attack me, I'd rather enjoy a 50% miss chance than a 5% miss chance, or better yet, I'd stay out of range and use ranged attacks, or simply make single attacks and move away so that it couldn't even find my square easily.
I like the idea of Unluck, trust me, I do. I've seen it cast. I can tell you, it FAILS in execution. Its not worth the action because it has a save. Almost every other save vs debuff spell at that level is better in nearly every circumstance.