Assume 80% chance to drop slot 1-5, 2% chance for artifact, 2% chance for insignia, 2% chance for ring, 2% chance for core, 2% for special roll (are uru special?)
So 80% base chance for slot 1-5 on first roll
Assume 1% chance for unique so .01 *.80 = .008 or .8% chance for a unique on overall loot roll
Increase RIF by 500%. Now artifact, ring, insignia and core are 12% (2% base plus a 500% increase of that base value, 100% RIF would be 4%), special still has 2% and slot 1-5 has 50% chance (the leftover amount)
.50 *.06 (higher chance because of RIF) = .03, alright so it's a higher chance than base
Now increase SIF by 500% as well.
Now artifact ring insignia and core still have 12%, special now has 12% as well leaving slot 1-5 at 40%
.4 *.06 = .024, now it's lower than it was before we added SIF
Now increase RIF by 1000% and reset SIF
now artifact ring insig and core have 22% each leaving slot 1-5 at 10% chance (SIF still has 2% chance)
.1 * .11 (again, higher chance because of RIF) = .011, now it's almost as low as base
now add in 500% SIF, ring core insig and artifact at 22%, special at 12%, that's exactly 100%, now you have a 0% chance at a unique
Obviously these numbers are made up (and I doubt slot 1-5 can get to a 0% chance) but if my probability is correct you can see how increased RIF and SIF could completely kill your unique chances (especially if your chance for cosmic items interferes with chance for uniques after you hit a certain threshold) if what Kanglor said is true about how loot rolls. With X-Def increasing your RIF (and SIF?) after each wave and bonus weekends, synergy, gear and especially boosts 1000% would be quite easy to hit if you're in a decent X-Def. In fact, the higher the base chance for any loot other than slot 1-5 the worse this problem is.
Now I just need for someone to come in here and point out that I am a math retard (I am) and this shit is all wrong. I can't imagine they knowingly designed a system that way but it sure as shit seems like it.