the only real negitive i've come across with barnes bullets is ,
if your barrel has any rough spots in it , it will copper foul much faster than it should , and your groups will open up .
if that happens you may want to fire lapp your barrel to smoothen things out . ......
also any fouling that wouldn't effect a lead core bullet may cause fouling issue's with the x bullet , you need to make sure your barrel is squeeky clean . the older barnes reloading manuals went into great detail on barrel cleaning .
also , because the barnes bullets retain so much of their weight , you can go to a much lighter bullet than a normal lead core bullet .
hornady also makes a copy of the x bullet , but out of harder gilding metal alloy , called the gmx .