I think the more general rule is that if your platform is advertising itself as an alternative to an existing dominant platform but that doesn't censor "X", the kind of people who'll flock to you will mostly be those who value X over any other interaction on the dominant platform.
Most moderates don't feel restricted enough in their speech to use Twitter even if they may have some racist ideas they don't feel safe stating there, but literal white supremacists who want to talk about the superiority of the white race all day long will love your alternative that doesn't censor them.
But if those white supremacists now all flock to your platform you likely now have a platform mostly consisting of white supremacists. This will naturally limit who else wants to join your platform because if they don't like being around tons of white supremacists they don't stick around long enough to build a counterbalance.
This can also happen after you think you've already established a more nuanced community as with your example of Freenet.
It turns out that while some people may seek opportunities to talk about X, others will not want to share space with discussions about X, often because of what it means to their own safety but also sometimes just from fear of association.
If you have a fun social network and it has a prospering community of nazis in it, you're now the nazi network, no matter how much of your community is about other things than being a nazi.