So what happens when a player that can have only great or awful luck tries to S-rank a dungeon that is laughably easy if you get good skyfalls?

No middle ground :V
1) Double B/G Karin, awesome skyfalls in every trash floor, literally ZERO skyfalls for 2 turns in a row in floor 5 causing me to waste turns pinging the shrimp, team rarity too high.....
2) Uh oh, all my Karin friends switched to A.Karin (which ironically is BAD for this dungeon), tried anyway adding a second mask to lower rarity..... skyfalls went on holiday, so despite oneshotting almost every floor (even floor 5 using Vamp to activate 12.25x) ---> lolnope
3) "F**k this shit".... A.Bastet friend and more masks..... skyfalls still on holiday (so back to shrimp-pinging) until Amberjack, buried under an hilarious 19-combo. FINALLY!