Actually, I've developed a method that works wonders without using zoom and supporting 3 different aspect ratios (4:3, 16:9, 16:10).
I'll see if I can come up with an example, but it IS a bit complicated. However, I've not had any problems with it at all.
If you don't feel like waiting for me to come up with an example you can see how I accomplished it by checking the cap file for my RPG game here:
http://www.scirra.com/forum/action-rpg-demo_topic45431.html
It should be under the "MainMenu" event sheet, in its own group called "Window and Aspect Ratio Settings". You'll need a SysInfo object and a Window object to use my method. You'll also want to check the "start of layout" actions below the "Array Initialize" group, as those use an automatic detection of the screen size to come up with a default aspect ratio. I also have the Project Settings set to have Menu, Caption, Resizing, Minimize, Maximize, and Fullscreen all disabled.
It's probably a bit more complicated than it needs to be, but it works great for me. I have no issues with scroll rates and only have to compensate for having additional aspect ratios.