re: So now it's racist to call Mexicans Mexican?

Differentiating work ethic can surely help put context into what you were describing. But what was the point in differentiating to someone who does not care? It's the undertones that are implied when you add those adjectives, even though you may not have meant any ill will. You could have said "some hard working Mexicans were still working on the house" and the person wouldn't have been offended at all.

It's ridiculous that people get so offended, I know.