The workflow is paradigmically different, and that may throw many people off.
There's no rebase, on principle. Branch names are permanent attributes of commits. Even abandoned branches remain visible forever. Code review is post-hoc.
People who are extremely accustomed to Git workflows may find the paradigm alienating.
Yep, it’s not for everyone, and their docs are pretty clear about that. OTOH those same docs contain a variety of arguments that git isn’t really designed for everyone either and that fossil might be a better fit for projects that have just one or a handful of developers, which AFAIK is the vast majority of projects on Github. My experience is that it is certainly an adjustment but the docs are good at explaining (and justifying) the differences, and the built-in features are very useful, especially for anyone looking to reduce their dependence on a centralized host that’s become unreliable.