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.
Thanks TW for the version reversion and Xenforo FTW!