Yep, economics. They used to even have their own phone, but soon moved to repurposing Xperia devices. Not sure if there was a relation between Ericsson and Jolla employees, but this would not amaze me.
Note: I have worked a lot on MeeGo and Maemo before this. I only had Sailfish running on my N950s, so can't judge the final devices. Wish I could ...
I'm using an Xperia 10 III w/Sailfish now, but honestly I liked the Jolla phone I had previously much better. Unfortunately the latter eventually developed an electrical glitch which made it useless and I had to replace it. The Xperia is way way too long, and with other physical things I don't like - but that was the only option if I wanted full Android support (as I had with the Jolla phone). And it doesn't handle the camera very well (it can take many seconds before it actually takes the photo, and there seems to be other issues too). It also didn't handle a Japanese SIM card very well, unlike the old Jolla one.
Other than that it works fine, but there's definitely room for improvements.
It would lose power suddenly, and restart. Battery was fine (replaceable), so it looked like a glitch somewhere in the electronics. But I used it for many years until then (had to give it up this spring).
Honestly the N9/N950 feels a lot more polished - UI wise - than sailfish.
It is kind of expected, given the budget of Nokia ... But even some of the design and aesthetic choices of Sailfish don't really age well once the novelty wears off
Note: I have worked a lot on MeeGo and Maemo before this. I only had Sailfish running on my N950s, so can't judge the final devices. Wish I could ...