Sub-domains are generally a bad idea for marketing and ecommerce.
Why? Because they cause fewer sales compared to using the root domain and with a much lower conversion rate.
Sub-domains cause all your other teams to work twice as hard because they now have to support 2 different websites. It makes analytics, ppc, seo, conversion and development all tougher and more time consuming to execute.
Sub-domains require cross domain analytics tracking. Many times, the sub-domain platform will not allow your company to customize the analytics code to allow the data to pass through to the main site. Regardless, to leverage a sub-domain, it requires a new analytics account, implementation, and customization.
Sub-domains make PPC more difficult too because you have to setup new adgroups or new campaigns. That’s because Google/Bing allow 1 url per adgroup. So to use a new landing page url with a sub-domain, you literally double your ppc work load.
Sub-domains are a separate website from the main company domain. So again, it doubles the work for the SEO specialist. In terms of SEO, it also affects your domain authority. Many companies use a blog on a subdomain – all that good content you are producing does nothing to increase your domain authority for your primary site.
Many a/b testing tools like click funnels, optimizely, instapage and unbounce are good tools with 1 main flaw – they force your company into using sub-domains.
In my experience, sub-domains never out perform root level domains. They are a burden on your marketing team causing everyone to work harder for no good reason.