I still use a Jolla 1 (JP1301) as a daily driver. With every OS release, some functionality breaks and some gets restored or improved.
As already mentioned, it’s no joy using the stock browser with any modern website. Even worse, many just don’t work at all.
Organizer tools (contacts, calendar) gradually get improved by community efforts (mainly @dcaliste) and get more robust plus more features slowly over time. With SFOS 3, I’d consider this to be quite robust from my experience.
With SFOS 3.3 (Rokua), they broke the Mail app, unfortunately: the app blocks the UI for seconds to minutes and SFOS offers to wait or kill the App. To read e-mail on a SFOS phone, you need to be very patient.
The OS and the apps tend to use more memory with each release, and as has also been mentioned, 1 GB RAM is no longer enough for true multi-tasking these days especially if a browser is involved. On Rokua, there must be some memory leaks in messageserver5, too, so this aggravates things for people tending to have a phone uptime of weeks. So you have to be careful what apps you need to have running concurrently – it’s no fun when your navigation app dies because you start the messages app to read a text message.
The J1 along with the Jolla C are the only phones still relying on the bluez4 stack for BlueTooth. With Sailfish X models, you’ll probably get better connectivity and have more options like MPRIS.
I can’t comment on Android matters, because I don’t have Android Support installed.
All in all, I think that the J1 may be the phone with the best and most mature overall OS stability, which is quite impressive, given its age.