Mastodon has such a different system architecture—at a business level—that I think many of the scaling challenges are entirely different. While my experience with Mastodon is as a (very satisfied) user, in general scaling challenges are as much figuring out how to scale the interactions between systems as they are scaling the individual systems themselves. That gets incredibly more difficult when those individual systems are spread out among various organizations with different priorities, philosophies, technical abilities, and resources.