It depends on what breed you're comparing with. EVERY breed comes with some genetically diposed conditions. Some have more and more serious ones than boxer, some have less, or less serious ones. And mixed breeds carry the genes of both parents, which may make them more or less at risk compared to their parents (all depending on the disease in question, and the breed of the parents). In short - there is NO simple yes or no answer to your question
I would suggest that you research which conditions the breeds you're interested in are genetically predisposed to - then spend some time finiding out how big the risk may be, and what breeders of those breeds are doing to prevent it. That gives you a MUCH better basis for deciding what risks you can diminish, and which you've got to accept (and finally, which breed you're going to buy). The following link is from one of the sticky threads at the top of this forum, via which you can see the conditions common in each breed (you'll note that in fact, boxers are generally prone to a fewer number of conditions than many/most others): http://www.upei.ca/~cidd/intro.htm