I've got a 200+ happy vault, 400k credits etc., been playing since launch.
When I realized that Luck gave you more caps when collecting, I started training my whole vault to 7+ Luck. I enjoyed massive amounts of caps when collecting. I went ahead and trained all 200 to 10 Luck, and now, 100% of my production facilities have workers with 10 Luck.
I get almost zero caps when collecting. I have 21-25 or so rooms generating, and can literally do several collection rounds and get nothing. It's as though having 10 Luck were as bad as having zero, and no collection bonuses (or extremely little) are being handed out.
Apparently I can't post links - but I'd be happy to show videos or images as proof. Having maxed out Luck means LESS return on caps. It's frustrating. I'm making a minuscule amount of cap rewards. I'd love to hear from others with maxed out Luck dwellers as well to confirm.