I've heard 301 redirects are SEO Friendly, but what about mass 301 redirects on a website for thousands of pages? Are there any SEO disadvantages of doing a significant amount of 301 redirects? The site currently has a PR 6 and thousands of back links.
If you are redirecting from old domain to new, use webmaster tools to preserve pagerank. Otherwise if you redirect thousands of pages to a single homepage on a new site it may cause some harm in the short term.
301 is designed for it, i dont think there is any disadvantages. however when you change your entire to work with a new domain you can expect to see some drop in SERP for a while.