Borg Defence, 1.......g5,

According to opening explorer for master games.  

Black wins 26% of games in replying to e4

However 1...g5 wins well over 30% for black  . It creates a fight and realistically nobody actually prepares anything to meet it.  Many would go their whole lives and never have g5 played against them.

  All the repetoire books have a half a page with some supposed refutation, but nobody bothers to read it, so white very much on his own from move One

familiarity is important but it can only get you so far.

at some point, what usually happens is that a strong enough player with enough time on his clock, will burn maybe 10-15 minutes more and still get a good advantage out of you. That's why an opening/defense must be half way decent to begin with, for it to be worth playing.

Relying on your opponents ignorance is rarely a good way to play good chess. IF you ever are pondering whether to play an unorthodox opening, put it through this litmus test:

"Would you be ok with playing this line knowing your opponent took half an hour to come up and familiarize himself with a computer reply.?". If the answer is no, then dont play it. the availability of hardcore chess analysis available at anyone's fingertips is too ubiquitous to play lines that can be refuted with some how before your match. 

This doesnt mean to always trust engines blindly, maybe that one position that stockfish gives as 0.8 scores respectably at master level, and a human may only think its about 0.5 in practice, but if you start getting -0.7's with white and 1.5's as black i would seriously reconsider adding them to a repertoire.

only 30 and 25 percent not a very reliabe defense