I used to worry about such a law suite back in my apartment landlording days; my buildings were not exactly on the most tony side of town and my asking for the late rents was enough to set off some of my tenants shouting charges of racism at me. Aw, the memories.
Then I attended some HUD/Housing Authority sponsored workshop for landlords and apartment managers. Our sponsors were all excited about the fact that they finally got a discrimination case to bring against a landlord. I knew the the area where the building was and could not figure out who the heck would buy a building in a low rent area and not know they would be dealing with people of all descriptions. It seemed to me if person had a trouble with such things they would never get a business serving the low income community in the first place. Someone pulled me aside and told me the inside story; It was a gay building that did not want a hetro couple to move in.
I stopped worrying about it after that. Yes, they are serious and yes there is a liability issue if you have idiot managers and yes it is easy to construe anything as being racist, but these cases are actually not all that common.