I have a game with relative sprite placement up on the Google Play Store, you can find the plug for that below.
It generally depends on the type of game that you are developing. My game is using the dimensions: 640, 960 (Scale Outer).
In my experience with relative screen-scaling, simple games like puzzle games for example require scale outer,
since scale inner often tends to cut off sprites on weird screen sizes.
If you plan to do a plattformer for example, scale outer wouldn't be the best solution since it shows more window content.
What this basically means for your audience, if you plan to use scale inner mode:
Bigger screens (more content shown) = Advantage | Smaller screens (less content shown) = Disadvantage.
You have tons of options to relatively design games, you could use:
- Anchor behaviors
- Startup viewport placement
- Cut off images with not so important content
I use a combination of everything inside my game, it might not be easy at first glance but once you've figured it out, it's fine.
Letterbox scaling isn't bad either, I'd generally recommend it for desktop (NW.js or non-mobile browser) based games only
but well, it saves a lot of work if you decide to use it. I hope I could help you out, you should wait for more opinions though.