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. If you have a commercial version of Sailfish with Android support, you might try Riot, but receiving VOIP doesn't work with that and notification is very unreliable, even with Android apps notifications installed. (I'm not pointing the finger at anyone here BTW.)

I have read elsewhere on this forum and on TJC that some third-party Sailfish app development has hit problems at the point that developer resources from Jolla are needed, and that this is at least partly to do with dealing with the closed-source parts of the OS. It seems to me that this specific issue won't be such a problem with the Librem phone.

I do understand of course that the Librem phone is some way from delivering, and like any project of its type it could fail. I also understand that for all sorts of reasons developer interest may be low, even if it does deliver. However Purism's approach is different from Jolla's in some important respects, and I would expect to see some differences in the user experience as a result, for example:

* Matrix support (as I have said)
* Sailfish's native e-mail client has some crippling problems with rendering speed that have really surprised me. I would imagine that Purism will do better.
* Being able to choose which search engines to add to the native browser without having to do any coding

This is largely speculation on my part of course and I might be proved completely wrong but I am very interested to see how it will work out.