Many thanks for your replies! Artpunk, those settings actually work very well on my iPhone SE and my 9.7 inch iPad pro when trying with remote preview. However previewing on a Samsung s7 showed some pixel distortion (the phone didn't show full screen like my iphone, so the preview was smaller than actual app resolution), and previewing in a browser window and trying out different sizes does show pixel distortion. This is mostly visible in small resolutions, where you can actually see some pixels smaller than others, while in bigger resolutions I can only notice a slight "waving" effect sometimes.
I can understand why this works on the iphone which has a screen resolution of 1,136x640px. If it scales to the window height 640, the window height 160 will resize nicely at 4x. What I don't know is why it works on the iPad with 2048x1536 resolution - i didn't get to see a full screen preview on that either, but Cluckle's Adventure looks just perfect! Maybe it's about the high resolution which minimizes the problem?
What yoyoEleanor suggested was something that I was thinking about too. How did you achieve the integer scaling?
Anyway, if Cluckles really didn't use anything more complicated than that, it seems like a safe bet to go with that kind of settings and no extra hairsplitting - the 3:2 aspect ratio does seem to work much better than 16:9! Have you found it to work on most smart phones?
Thanks AllanR for your example too, I'll keep all of that in mind! The example had linear sampling which gives a different look, though, and if I use small sprites they get too soft.