Google's John Mueller said on Twitter that hreflang "is one of the most complex aspects of SEO (if not the most complex one)."
If you set it up for a couple pages, maybe not but most of those who implement hreflang do it on large dynamically driven sites on large CMS platforms. It brings together everything that can be complex about large dynamic sites and then just amplifies them but adding in more than one language and/or region.
There is no doubt in my mind that John is correct here. The concept of hreflang is pretty simple but implementing them, espesially for the sites that often need them implemented, can be a huge complex challenge.
Here is John's tweet:
Forum discussion at Twitter.TBH hreflang is one of the most complex aspects of SEO (if not the most complex one). Feels as easy as a meta-tag, but it gets really hard quickly.— John â.o(â§â-½â¦)o.â (@JohnMu) February 19, 2018