It's at high risk to run in the medium-reputation-problem: If you have e.g. the choice between your stuff being linked as "foobar.platform.com" and "foobar.com", the former is great if the platform confers a positive image over any other random URL. That's often the case early-on for new platforms, when they pull in cool early adopters and thus are associated with high-quality content. E.g. the trope-namer medium.com early on had a reputation for nice clean pages and good content. The problem is, that often changes over time, and now a medium.com link encountered in the wild suggests "stupid subscription nagging" and "low-quality self-promotion think-pieces", it's not the neighborhood you want your content to be seen in if it is any good.

For now substack isn't there yet (although you start to hear the first sentiments in that direction), and their focus on subscription helps avoid it to a degree too, I also see more often people point to "their substack" from elsewhere. But if they go the path of putting the platform more in the foreground, i.e. not "foobars newsletter, which happens to be on substack, but you don't really need to know or notice that", but more "go to substack to find cool content!", that risk increases. Again the medium-comparison: medium was and is big on finding other stuff on medium. Which means even if you read a good article, right next to it medium will push you to read more fluff, because their algorithms have no clue how to promote relevant high-quality content.