I wouldn't say that web dev is "hard" as in any part would need a very deep knowledge, but it sure as hell very wide.

There are so many things and concepts, multiple must-know languages, browser quirks, some networking knowledge, CORS, etc. If you do use an "industry-strength" backend framework then the complexity surely drops, e.g. it will handle injections and stuff, but not having heard at least a bit about what your framework does for you, and reinventing the wheel can go really bad really fast.