If you're worried about it breaking, then you got the wrong harness. I have a leather harness I've had for years. It sat in the bottom of a box forever. Pulled it out this year, oiled and saddle soaped it up and put it on a pony after years of sitting. It did just fine and was very flexible after one use. Just wanted to add, I've had this harness for over 10 years and I bought it used then. So I'm guessing this leather harness is 20+ years old. Has some wear and such, but its still a great peice.
If there are visible cracks, then take it in to a leather dealer and have them re-enforce stitch certain areas (I did this after I did have a horse break a harness becuase they flipped in the cart). But most good leather shouldn't break on you. Or sell if and get a better harness.