I tested C3 with an empty project (just background color) on some old devices and these devices just show black screen.
* Samsung GT-S7580 Galaxy Trend Plus (only default browser) Android 4.2.2
* IPhone 4 (safari) IOS 7.1.2 (11D257)
And I found the problem, it is in c2runtime.js file in line 679
Just change let to var.