SwiftPaws has been designed for durability. The string and flags are completely replaceable, even if your dog were to chew up a flag or chew through the string, you simply tie it back together and you're good to go again. Catching the flag is part of the fun!
As for the pulleys/machine, we wouldn't recommend leaving your dog unattended with the equipment. Pulleys and machines are not chew toys! However, if your dog steps on, or disturbs them during use no worries! They have been made to withstand a good deal of play.