In recent August 28th Google hangout on Google+, John Mueller indicated a very important fact by saying at 18:30 mark that Google may not give HTTPS web pages the desired HTTPS ranking boost in case the page serves up HTTPS content mismatch errors.
What does it actually mean? It’s relatively simple, when your web page uses HTTPS protocol but somehow there are some elements on the page that are not secure e.g. images not over HTTPS or any social media plugins etc.
So far Google has been quite liniment and your pages might have benefited from move to HTTPS, even if they are not fully HTTPS secure.
However according to John Mueller this may change moving forward;
I imagine at some point, we will say you will have to serve really clean HTTPS for us to show that in the search results. I don’t know. I don’t think that there will be any kind of penalty around that. Just that we may say, we won’t count this as HTTPS, so it won’t get that small HTTPS ranking boost.
Summarising, think twice when planning to move to HTTPS as many of my industry piers have been already noticing site visibility drops after move to HTTPS even despite keeping all clean and tidy in the process. I’m still of the opinion that there always should be strong business case to support move to HTTPS rather that doing it for the sake of compliance with Google’s advice.