Well, Purism have hitched their wagon to Matrix ("Purism is making the Librem 5 the first ever Matrix-powered smartphone"), so they are presumably going to make some effort to support a broad set of features, even if a feature-complete mobile client isn't feasible. With Sailfish, there is nothing available that supports (for example) VOIP on Matrix, and little prospect of anything much appearing in the foreseeable future. Even the new Transponder project won't.