Consistent and robust options to convert ICNS files to PNG in macOS?

I've been having a terrible time getting sips and/or ImageMagick (convert) to consistently convert .icns files to .png. I frequently hit very unclear errors when using sips to make the conversion (eg, sips -s format png "/path/to icns file/file.icns" --out "/path/to png file to generate/file.png") and when I end up with problems with that, I have usually been able to use ImageMagick (eg, convert "/path/to icns file/file.icns" "/path/to png file to generate/file.png"). I'm suspecting there are inconsistencies in the .icns files that I'm attempting to convert, but the sips error/output messages are sometimes nonsensical and unhelpful in this regard. When I hit these issues I can usually use ImageMagick, but not always. Has anyone come up with a reliable method for converting .icns to .png?