Sailfish is a Linux-based mobile operating system primarily targeted at mobile phones.
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Pro Unique gesture-based interface
There are no buttons needed to use it.
Pro Easy to use Swipe UI
Pro Lower power consumptions
Pro Inbuilt Terminal app and ssh essentials
Smartphone for power users, not just for users.
Pro No annoying search bar that you might not be able to remove
You can actually manage your files. The third option to iOS vs Android.
Pro No pre-installed Google services
But if you want, you can install it.
Pro Usability and privacy
No private data transfer to other companies.
Pro Very customizable
You can use patches to change UI or other stuff.
Pro Nearly monthly updates and security fixes
Con Very old packages
It uses a fork of QT5, Wayland and other software. So developing and running software for Linux is very hard on SailfishOS and back, running apps for Sailfish OS on other distros is complicated too.
Con Has proprietary UI component
This would lead to unwanted security compromising bugs that may not get enough eyes to be marked as a bug.
Con Only a few native apps, needs Android support to access many apps
Users may need to install Google Play store or other alternative android app store to access your bank's app or any highly specific app you might need.
Con Only a few available phones
There are currently only a few available phones on the market with Sailfish OS and both the Jolla and the Jolla Tablet are out of stock and unavailable.