Where you follow the given steps carefully and one fails because whichever version of something is not the right one, then you end up in a weird state where you have a hard time going back to how your setup was before, and you have to google things like "Compiler error: missing dependency foo.h" even though you can see that foo.h is right there. You then finally fixed the foo.h error to discover that you need to update Ruby for whatever reason, but the system doesn't pick up the version you installed with Homebrew anymore but is using the old default one, you then start updating Homebrew. Etc., etc. until you don't even know what you were trying to achieve in the first place and you are now super irritable.
(yes, I had a recent experience with that kind of yak shaving :))