Ahrefs recently conducted the largest hreflang study to date, examining issues on 374,756 domains that use hreflang tags. The study found that 67% of these domains have at least one issue. The most common problems include: 56.3% of domains have pages missing x-default, 18% have pages missing self-referencing hreflang tags, 16.9% have hreflang tags referencing redirected or broken pages, 15.3% have pages missing reciprocal tags, 8% have hreflang tags pointing to non-canonical URLs, 4.6% have pages with incorrect hreflang values, 3.2% have pages with inconsistent language attributes, 2.5% have more than one page referenced for the same language, and 2.5% have the same page referenced for more than one language. The study highlights that hreflang implementation can be complex and prone to various errors.

source: Over 67% of Domains Using Hreflang Have Issues (Study of 374,756 Domains)