Sites Without Equivalent Mobile & Desktop Content Won’t be Switched to Mobile-first
If Google judges that a site’s content isn’t equivalent on mobile and desktop then it probably won’t be switched to mobile-first indexing.
Desktop Search Results Will be Generated from Mobile Pages with Mobile-first
Desktop users will see relevant desktop pages, but the search results will be generated from content crawled from mobile pages after mobile-first indexing. Desktop ranking will be impacted if you have reduced content on your mobile pages as that is where Google will be sourcing content from.
Provide Desktop Version to Mobile User Agent if There is No Mobile Equivalent
For mobile-first indexing, if there is no mobile version of a page it should not 404 for the mobile user agent. Instead the desktop version should be made available for Googlebot Smartphone.
Separate Mobile Sites Need to be Crawlable at the Same Speed as Desktop
Separate mobile sites need to be able to be crawled at the same speed as on desktop so if your separate mobile site is hosted on a slower server then this will affect Google’s ability to rank it after mobile-first rolls out.
Sites With Poor Internal Linking on Mobile Probably Won’t be Switched to Mobile-first Indexing For Now
Internal linking is taken into account before Google switches a site over to mobile-first indexing. If a site has terrible internal linking on the mobile version of the site then that site will likely not be moved over just yet.
Hreflang Tags on Separate Mobile Pages Should Point to Mobile Equivalent
Different language versions on separate mobile sites should have hreflang tags pointing to an equivalent mobile page not an equivalent desktop page.
Use Same Hreflang Markup On Separate Mobile as on Desktop Site
For mobile pages, Google recommends having the same hreflang markup as on desktop pages – they should be equivalent in terms of metadata and content.
Incorrectly Configured Mobile Sites Show in Desktop Search
Migrate From Separate Mobile to Responsive Before Mobile First
If you are migrating your separate mobile pages, set up 301 redirects from your mobile URLs to your canonical pages. Rankings should not be affected before mobile-first indexing, however you might see more fluctuations after mobile-first indexing as your mobile URLs will be the canonical pages, and it will be seen as a site migration.