I cannot tell you how many times I speak to people about site migrations or mass CMS changes to their web site and they tell me, we are just going to 301 redirect all the internal pages that no longer exist to our home page - so our home page will rank for those keywords. I tell them, it doesn't work that way.
Well, John Mueller of Google went on record that when you do 301 redirects of many or all of your old internal pages to your home page, Google will treat those redirects as soft 404s. Which means, essentially, Google will ignore those redirects, not pass the PageRank and signals from the old pages to the new page.
He said this at the 4 minute 52 mark into the video. He said:
So the the 301 redirect from all pages to the home page, that would be something that we see as a soft 404s.
Here is the video embed:
