We have 6 sub domains which are forums, guides, etc. They have their own visitors for the related queries. We are planning to divert some of them to the website to promote our product with latest content. We are planning to add a link from every page of sub domain to our website homepage. This makes additional thousands of internal links flowing to website homepage. Will this kind of internal linking structure hurts? Any risks involved? I heard like too many links to website from root domain is not good as per Google.