Very true in the sense that they are a great way to gain proof of concept for a novel drug early in development. But the vast majority of drugs that work well in vitro with cell cultures don’t work in vivo as a living creature (mouse, dog, human) is vastly more complex than a cell culture plate. Additionally, most of the popular cancer cell lines that are used in labs have been selected to be highly responsive to drugs to give research the greatest chance at success. I worked with CBD in a cancer drug development lab and it worked incredibly at killing cancer cells in vitro, but once you tried in an animal model, the effect size shrank dramatically and vanished more often than not.