The phrase "vast oversimplification" means making something much simpler than it actually is, which can lead to important details being overlooked or misunderstood. Full definition
It is a vast oversimplification and it's dangerous to ignite class warfare and cast every well - off American as greedy and selfish. (religion.blogs.cnn.com)
San Francisco is the place with fog and Oakland is the place with sun, sure, but that's a vast oversimplification of the 39,941 different microclimates in the Bay Area. (sbnation.com)