The issue is with things like Qt versions, component sets, and system APIs.

Ubuntu uses a newer Qt framework version than sailfish does, and so not all APIs are available that might be used in any given ubuntu touch application.

The next thing is component sets, apps are typically built with the native component set and in the case of Ubuntu Touch and Sailfish they are quite different and so chunks of the UI might need to be completely designed if you want a native look and feel.

Then there's some more platform specific stuff like multitasking, notifications, power management, that work differently on Ubuntu Touch and Sailfish that you will need to take into account if you want the app to work well.

From what I can see, the *real* issue isn't the work required, but its people with the skills giving time and effort that is currently somewhat lacking in the Sailfish Community right now.