Etcher documentation says:
Etcher copies images to drives byte by byte, without doing any transformation to the final device, which means images that require special treatment to be made bootable, like Windows images, will not work out of the box. In these cases, the general advice is to use software specific to those kind of images, usually available from the image publishers themselves.
The image is copied to drive byte by byte without any modification. If your ISO image isn’t hybrid (doesn’t contain partition, …), it won’t boot from the USB drive. I’d recommend to use Google with the bootable USB drive from an ISO image phrase. Lot of results / ways how to do it on Linux, Windows and macOS.
I also did ask the Etcher team if it’s something we have on our roadmap or not.