In practice it takes a long time to build a config for absolutely everything in your system at least from my experience of it
There are a few things I've not bothered to put in the config like wallpaper, gtk theme, macros (though I do want to put macros in declaratively because it feels like missing a limb not having them)
System setup wise you can completely wipe the machine and have it back up to where you left it in 20 mins or so