Rankings and rigor, with a few exceptions, mainly due to happenstance, have no relationship. There are a very small handful of tech schools who are known to go the extra mile on theory. You could include Caltech and MIT in that group. They, particularly Caltech, primarily based on volume of material and the speed at which it’s taught, will then be more difficult.
As for the rest, they are all over the map. Some schools that are lower ranked have significant engineering attrition because they are hard and they don’t hand hold students to get them through. This in turn lowers their ranking.
Know this, no engineering program will be easy. Beyond that, it’s a crap shoot. You can’t infer anything about difficulty from ranking.
I’d go one step further, be careful about inferring ANYTHING from school rankings, especially USNWR.