Luck DOES affect item drop rates. Did a few play tests already, two 6 hour ones, and one 12 hour. What i found is that luck essentially point for point affects almost ALL drop rates at a 1 to .001 rate. But the drop rate is affected BY ALL the luck of EVERY party member totalled.
For clarification, some items have a fixed drop rate, or only drop during certain fights in certain areas, like with antiques. These also are somewhat affected by drop rate, but only minimally, as they're common enough already.
there are several things that make me confused between dex and luk. Dex is certainly related to accuracy / hit rate (made clear by Yaelu whose attacks often miss because of low dex) Luck between critical or stun rate, because Lian has high luck and she often stuns enemies, and Lian is more often critical than Nowa. On the other hand, Mio also often criticals with her high dex. So critical is based on which attribute?