I have two completely maxed out dwellers who don't find much of anything. Normally they'll come back with a lot of stuff but almost none of its good or useful. They can both stay out there for a couple of days, 3 at the most. But they never find power armor, do I have to find it in lunch boxes or can they find it.
They can find it. I witnessed myself that they can ;). As you propably know, Luck and Time stayed out, are the main factors for good gear. You propably just had bad luck.
My Experience is, that they start to find the good stuff after about 10-12 hours outside. Just equip them with your best gear and lots of Stimpacks/RadAways and they will find precious stuff for you.