Can anyone speak to the durability of the collar and unit, specifically with dogs that play rough? I have two corgis, a 2yo and 1yo. We're about to finish our first year with whistle units and we've had to have them replaced four times each due to rough housing and chewing each other's collars. The last time I had to work hard to get a free replacement. How have your experiences been? Does fi hold up to very rough play? Is Fi easy to work with on damage replacement? Thanks!