Your question is mostly open to opinions and I don't think there is a clear answer. You'll have to evaluate each framework and decide what is best for you.
Both FreshMvvM and ReactiveUI are MvvM frameworks. They don't impose a distinct limitation on what kind of project you can make unless they don't support a platform that you need. Consult the respective sites for each framework to see which platforms are supported.
ReactiveUI
FreshMvvm
It's also worth mentioning that ReactiveUI takes advantage of Reactive Extensions. It encourages "functional" style programming vs. imperative.
I don't see any reason why either MVVM framework would limit the type of application you can make as long as you have the supported platforms you want. These are just frameworks to help with the design patterns of MvvM*.
If you can ask more specific questions about your hesitations, that would be great.
Note* FreshMvvm was designed with Xamarin.Forms in mind, while ReactiveUI supports other non-mobile platforms as well.