There Isn’t a Separate Index for Mobile and Desktop Indexing
Google have one main index where either the mobile or desktop version of a site is contained, this is the version which will then be shown in search results. However, if you have a seperate mobile site, Google will always show this version to users on a mobile device.
Mobile-friendly Errors on Desktop Page Could Mean Google Doesn’t Understand Connection With Mobile Version
If mobile-friendly errors are reported on for the desktop version of a page which has a rel alternate to a mobile version, this could mean that Google doesn’t have a clear understanding of which pages belong together and they could be indexed separately.
You Don’t Need to Submit Separate Mobile URLs in Sitemaps
URLs for separate mobile sites don’t need to be submitted in their own sitemap. If the rel alternate tags are set up correctly then Google will be able to find these pages through their connection to their desktop equivalents.
Poor Internal Linking on Mobile May Impact How Google Indexes and Ranks Pages
Poor internal linking on the mobile version of a site could impact the way Google index and rank the site after it has been switched over to mobile-first indexing. You can use tools like DeepCrawl to crawl your site with a mobile user-agent.
Move Disavow File to Your Separate Mobile Site for Mobile-first Indexing
In the same way that you should move the disavow file across after a domain or HTTPS migration, you should upload the disavow file on your separate mobile site if it was previously uploaded just on your desktop site.
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.