Hard to learn
As I am reviewing one of my old Korean books I am reminded of all the things that textbook authors do to make it hard to learn languages.
Most textbooks introduce vocabulary in categories. The names of the colours , or the parts of the body etc. will be introduced together. Words that mean similar things or opposites will be introduced together. So the vocabulary list for each lesson includes no only what I need to understand the text in a given chapter but a lot of (as yet) unnecessary words that the author thinks will help the learner.
In fact this does not help. Research has shown that it is more difficult to learn new items in such categories or associations. We end up confused. The brain does not remember them.
We learn best when we can get a clump of information, or a few new words or phrases together with a lot of familiar material or content. If we know all about flowers and are reading about flowers then a few colours or names of flowers can be absorbed as part of reading and hearing about interesting content.
A long list of the names of flowers or colours, on the other hand, is very difficult to learn. Most facts or concepts that are taught in a way that is separate from real and interesting language content is not easy to remember. That goes for grammar explanations as well.