To what extent is "English usage" a controversial issue from a linguistic point of view?
The issue of "English usage" is controversial to the extent that there is conflict between the ideas of prescriptive linguistics and descriptive linguistics.
Many linguists only do descriptive linguistics. They want to describe how languages are used by those who speak them. They do not start with the idea that there is some proper way to speak the language and try to encourage that usage among the populace.
However, there are some (and even more among the non-linguistic community) who want to practice "prescription." These are people who feel that there is a "pure" form of the language and that people should adhere to that form.
The issue of "English usage" then, is controversial because some say that there is a correct form of English that should be taught and privileged over others. At the same time, others argue that the idea of a correct English is something that is used by a particular group of people to try to maintain their cultural dominance in their societies.