External Link Signals Are Only Passed Between Domains & Subdomains via Internal Linking
Signals from your external links can be passed via internal linking between a domain and its subdomains. These signals aren’t shared automatically to the main domain if someone links to your sub-domain, for example.
Redirect Old URLs When Migrating Images to a New CDN
When migrating images to a new CDN, make sure to redirect the old image URLs to the new ones as well as updating the embedded links in the webpage. Images aren’t crawled as frequently as webpages so you need to make sure all signals are aligned so Google can move them over as quickly as possible. Consider setting up a subdomain and transferring the images, so the URL doesn’t change except from the hosting.
Microsites Can Be Seen as Doorway Pages
Microsites often look like a collection of doorway pages. If you are looking to build these microsites up in the long run then this might be an option, but if they don’t have value beyond driving traffic to another site, then microsites aren’t recommended for search and should be noindexed.
WWW Subdomain Should Not be "Parked"
If the www version of your domain is "parked", it might affect the indexing of the other versions of your domain.
Subdomains for multiple sites
Subdomains are treated as separate sites, or part of the main domain depending on the signals.
Google Recognises Separate Sites on Subdomains
Google has a process to recognise when separate sites are hosted on subdomains, and when they are used for a single website. They will then apply per site e,g, penalties and malware detection, but this can affect the entire domain if they haven’t recognised them as different sites.
Wildcard Subdomain Configuration Causes Crawl Issues
Using wildcard subdomains can make a site difficult to crawl.
Sub-Domains May Be Treated as a Separate Site
Sometimes sub-domains are treated as independent websites, depending if Google can detect they are related or not. So a closely linked blog on a sub-domain is going to be OK.
Only Manual Penalties Show in Search Console
Only manual Google penalties are shown in Webmaster Tools. They can be applied to a single subdomain, or all subdomains of a domain. They are removed automatically after a fixed period.