So it's actually a screen aspect ratio problem than a resize problem? I think there should be some ways around that... I havn't looked in that that much yet, since I'm not that far in to the production yet, but yes that's an issue i have to deal with as well at some point. If you're making a game in 16:9 format and displaying on a 4:3 screen there will be black borders under and over, but I don't really see it as a huge problem since most smart phones today are in 16:9 format...
Iphone 4, and iphone 5 differs in ratio for example, but if you really want to optimize your game for phones with aspect ratios other that 16:9 i don't think it would be too much trouble, just have to be taken in to account when designing the game, especially the UI. But i wouldn't worry about it too much since most smartphones use a 16:9 ratio. If you want to please a few% that has other screen ratio then i think it can be done properly without too much hassle.
Scaling a 16:9 game to fill a 4:3 screen vertically without black borders might crop out stuff that are to the left or right, (Probably mostly UI Stuff) but that can probably be adjusted for with events.
https://www.scirra.com/tutorials/73/supporting-multiple-screen-sizes
UI elements can be placed using these system expressions instead... if you want something to stick to a certain point, like the lower right corner you could probably use set position to: WindowWidth - 30 windowHeight - 30, that would probably work for any screen ratio.