Old sailfish user here, currently on an iPhone. The following things are why I switched.

• No support for Bluetooth LE, either natively or in the Android emulation. (So no smart watches, bike computers, etc.)
• Bluetooth headphones worked fine. Mostly. And not with any good codecs.
• The native app ecosystem was small. You will need to fallback to android emulation for a lot of things. In particular, good luck getting any commercial apps natively (trains, banks, and so on).
• The native apps that existed were clearly hobby projects, mostly written by one person for the use of that person. Which is fine, but none of the apps I used left enough of an impression for me to remember them.
• The web browser was fine to use, but absolutely not up to standard w.r.t. security (at that time). Plenty of known issues in the underlaying engine.
• No decent native app for maps. I never tried a native app that managed over 15fps when scrolling around. I think I also had issues with position data in android apps…
• Finally, I was constantly switching between two very different user experienced - emulated android and native sailfish.

Native sailfish was absolutely wonderful. It worked flawlessly when I stayed within the fearures that were available natively. But in modern society you simply can’t get away without all the android/ios stuff. At least not unless you have a lot of time and energy to fight the system, and I currently don’t have that.

This was a few years ago, so things may have changed for the better. I’ve been busy and haven’t kept track.

But if you want something to do - write an amazing maps app for native sailfish or get the Bluetooth LE-stuff working.

This project was started recently-ish and implies reverse engineering is the most important part. https://librephone.fsf.org/faq Haven't seen much else listed out.

I’ve never seen a roadmap

Neither have I, sounds like a good project in itself if it doesn't exist. 'Drivers for xyz, reverse engineering something' is part of the problem, phones usually (nearly always best I understand) have proprietary blobs of firmware to a greater or lesser degree and it's a moving target different between manufacturers and most often models. Qualcomm modems are particularly egregious for patent reasons. US trade deal enforced global DMCA laws make reverse engineering legally tricky. Hence the desire for linux specific hardware platforms.