Subdomain vs Subdirectory for SEO: A 2017 Case Study - GrowthHackers

One thing I've seen with this debate... 90% of the time this is related to the blog,, since almost every site has a blog.

When your blog is on a different CMS than the rest of your site, sometimes the performance is worse. That's because marketers need a quicker way to publish content.

Well if your blog is on a CMS it's probably going to be slower and have way less SEO related features than your main site (AMP, schema, internal links with diverse anchors). Also your site structure or permalink structure may not be ideal and you'll probably have lots of other pages in the index of value that also contribute to kw cannibalization.

So a lot times it makes sense to have it in a subdirectory because you'll see: -better internal linking -performace -consistency with site structure -consistency with SEO related page elements -less risk of kw cannibalization

Sub directory is usually better. Also links pointing to your blog are worth a little bit more.