With their six-cylinder engine, head gasket failure is almost unheard of.
I had a 2002 six-cylinder Outback, and never had any problems relating to head gaskets–or anything else for that matter–during the 10 years that I owned it. My 2011 six-cylinder Outback has been similarly problem-free.
I have to wonder about the head gaskets that your mechanic used, and I also have to wonder if he torqued the heads correctly.