Okay, I took one for the team (not really, but it's fun to be a little dramatic): I got a sense of Android applications under SFOS by getting an Xperia X -- a really cool phone, btw -- and purchasing (50 euros) Sailfish X for it. Yes, it was a little bit of a pain. To purchase it, unless you're in one of a few favored countries, you do need to d/l Opera and select one of its VPNs that emerge in one of those countries. Then you can download it. Don't kill your Opera VPN session just yet, though, because the links in the fairly complicated flashing instructions, that require an additional download or two, also fail unless you're in one of those countries.
That having been said, flashing was fairly straightforward. Next up was installing Open Repo (just because), F-droid, and Yalp Store. Whence I was able to get the ProtonMail for Googledroid application that was the entire purpose of the exercise.
Having lived to tell the tale, I can say that Android apps pretty much work on the commercial Sailfish, so presuming there will be one for Gemini, we have that to look forward to.
I say pretty much because there are a few annoyances. I have not, for instance, been able to figure out how to get notifications from the ProtonMail app, nor do I know whether it is running while its GUI is closed, as it is on every other platform. One can download mail attachments, though I have no idea where they go. They do not go to /Nemo/Downloads.
And there is still no WiFi calling, meaning that in telephony the latest and greatest Sailfish OS is behind Blackberry's BB10 of 2012.
That having been said, it is a great phone OS, as you know from Gemini, and the newer version, with Alien Dalvik and other new stuff, is even better. For now, the Xperia is my daily driver. Got to set up the VPN on it and it will be good to go.
