Scale letterbox will expose your 'outside layer elements' if the target device has a bigger/unfitting resolution then your project view resolution.
Scale inner scales to fit to the smallest side, meaning you could end up having a part of your layout outside the users target devices viewport.
Stretch to fill actually stretches your project view to adapt itself to fit in the target screen , taking on its ratio, often problematic for objects placed in a HUD.
I set construct generally scale outer, and have about 20% of extra background around my viewport to fill empty spaces from resolution/aspect ratio differences.
This way they will just see a bit of extra background and no black lines.
This means it will scale the size of your project view until its widest size matches the width of the target device.
This is as close to an easy solution I got for an approach of 1 fits all.