I don't think they are that much harder than 'regular' balls. I bought 108 of them in 2010 when I bought my ball machine and used them exclusively in it for two years hitting for probably at least 1 hour per week. I never had any arm or elbow problems from that. They are somewhat harder than pressured balls but they aren't like hitting rocks as some people are implying. The only reason I don't use them is that the outer felt finally wore off and they're pretty expensive to replace. I created a system to repressurize regular balls and now use it to keep my ball machine supplied with good balls.
When should you serve and volley in doubles? Only when you want to win.