True story:
Years ago when Amex was matching 150k offers, I called Amex to get matched.
I had gotten like 3 different cards from Amex in the 3 months prior to the phone call.
I got a call from Amex a day later from "the rewards abuse team", basically telling me that I'm an abuser, and I will get shut down if I ever open and cancel a card again.
It was a legit call - because the guy spelled out all the abuse that took place.
I've been a regular DDFer since then, and nothing ever happened to me.
Remember this was a long time ago.