I just did a Luck test to see if Luck affects how quickly you find items, or the quality of items that you find, and it doesn't seem to affect them. One dweller had natural 10 Luck, the other dweller had natural 10 Luck with +5 Luck gear, for 15 Luck, i sent them out within 2 minutes of each other. I frequently checked them, and counted the amount of items each had, and it was roughly the same, within 1-2 items of each other. Luck definitely does affect how quickly they collect Caps though. By the time the 10 Luck Dweller had collected 6,000 Caps, the 15 Luck Dweller had collected 9,000 Caps.