When comparing feh vs nomacs, the Slant community recommends nomacs for most people. In the question“What are the best FOSS image viewers for Linux?” nomacs is ranked 1st while feh is ranked 3rd.
Ranked in these QuestionsQuestion Ranking
Goes well with tiling window managers.
Pro Loads up images faster than my imaginary girlfriend replies to my texts
Pro Highly customizable
Pro Detailed view of image metadata
Starts very quickly and has a small memory footprint.
Pro You can work with hot keys that can be reassigned as you want them
Pro Simple and pleasant look
No cramped toolbars or large icons; the UI is cohesive, minimal and keeps out of your way.
Pro Frameless view
Pro Lots of Image Adjustment Tools
Pro Allows to cut (frame) images
It is important if you have to work with photos of documents and want to cut off the background on which you made photos.
Pro Flexible thumbnail views
Can display thumbnails in a strip or a zoomable grid as well, also has hotkey for both of these. Opening a folder is one of the main actions on the toolbar!
Pro Remembers last used folders
Often used folders (e.g., screenshots) can be quickly accessed.
Has builds for Windows, Mac and Linux.
Pro Batch image processing
Has tools for applying conversions etc. to many files at once.
Pro Comparing two images
This feature is called Synchronization. With the synchronization it is possible that multiple viewers perform the same action (like panning, zooming, etc.). This feature is useful when comparing two images. See more here.
No way to get to the next image in a directory from the program interface.
Con Only partial support for HEIC
Apple's HEIC format is only partially supported. Nomacs can open individual images, but can't navigate among them.
Con Heavy compared to other Linux image viewers
Uses more memory and CPU than other lightweight image viewers. High CPU usage when starting.
Con Returns to default settings after update
Con Recent version has slowed down
The latest version (3.8) suddenly became very slow in Fedora 26. Slow to step from image to image; slow to open a directory; slow slow slow. Downgrading to the previous version (3.6) restored speed.