1 - the native browser being an old firefox/gecko fork embedded into their own UI framework, giving a poor performance and dated compatibility quirks 2 - the android emulating runtime meant that you get again, dated , poorly performing android apps, that you're driven towards because the browser engine was so poor.
these two mean you basically end up with a sub-standard android handset/UI, and a tiny market for native app development (because everyone made do with android), its a real chicken/egg.
In fairness I've not used it since the sony XPeria days, but it was my daily phone for 3-4 years since the Jolla 1. It was cool being able to emacs and irc natively on the phone, but that was limited in use cases tbh.
Also permission model on Sailfish was much worse than on Android. I didn't use Android apps on Sailfish, though.
I really liked Silica UI, but available apps had much less functionality than their counterparts on Android and iOS. I think that open sourcing Sailfish and Silica would end up better for them.
Nevertheless, I kinda liked the phone, but ultimately went back to Android.