well mostly people make a guess without any bases. sometimes they say it just because BTG is not advertising enough, sometimes it is because it has a premine, and sometimes it is because they were planning on running an ICO but changed their minds.

in the end, the question really is what do you call a "failure"?
something that has bugs and people lose money because of it? then ETH is the biggest one since people lost millions in it!
something that has a low price, or declining price? then all the altcoins that ever got pump and dumped are failures.

BTG will happen, and i think it can even gain some popularity and even get pumped. but i don't expect anything big out of it.