The main counter-argument to this I have read is "you should just publish blog posts instead." This is, among other reasons, because

- The bar to releasing the first thing to the public is lower,

- The regular cadence builds an organic following,

- You can iterate on subjects in the open,

- You have more freedom to organise content non-linearly.

The main drawback I can think of is you cannot call yourself "author of book X". Are there any other reasons not to blog instead of book?