This works by reducing the colors down to an 8-bit palette. If one does throw a very colorful image at it the quality loss visible. So don't overwrite original/master files, it not lossless. But the algorithm used is indeed VERY good. Attempting to do the same thing manually in GIMP (Image > Mode > Indexed ...) creates much worse results.
With the GUI-based pngyu I ran into "Error: Process timeout" on large files if the quality was cranked up. It's based on
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. Looks like Pngyu was last updated late 2013 but pngquant is still updated on github.
It is possible to download an updated version of pngquant replace the old binary in pngyu's sub-folder. That also got rid of the "Error: Process timeout" for me.