"Spreads like cancer" not so much to me. "Is cancerous" I would say to avoid saying, unless it legitimately IS a risk for cancer.

I was told that being in a brick building, is not a threat for cancer. However, living in a brick home after say 40 years, actually CAN be a risk for cancer. Which not a lot of people would know. So I think keeping "is cancerous" to an actual legitimate basis is a good idea to spread actual good information. Because if I said "brick houses are cancerous", that sounds like an exaggeration, but in the long run it's true.

"Is cancer" really just depends on how casually its being used. Something that causes you a mild inconvienence? Yeah, I'd find that offensive. But using it towards something that may not be literal cancer, but still poses a physical harm, I find less offensive. Like saying "Nestle is a cancer on this planet". I wouldn't find that offensive, because Nestle uses literal slavery, while draining the planet of natural resources, and giving the middle finger to everyone else. Yeah, THAT can be called a cancer. But youtube? Put on an adblock, move on with your day.