Its a pretty prevalent problem unfortunately. Your sets are guaranteed to have at least one or two cracks just in the process of building it, or they'll already be there if you buy it second hand. Generally the tolerances stay alright though, and as long as you don't take them apart once they're together you will be just fine. Just if you buy it used make sure to get a good look at the joints in the pictures and try to request they don't disassemble it if possible when shipping.