Of course! Rangers/Berserkers/Glacial/Poison, or 6 Mages, or 6 Infernos, or Lights/Summoners, or Yasuo, or Crystals, etc - I've seen many different builds win the game. Especially when multiple players compete for the same powerful build, while somebody else has nobody competing for their champs.
You've only lost a game with 6 Shadows once? I guess you just haven't played much against serious opponents (lolchess.gg says you only play non-ranked). Assembling 6 Shadows is far from trivial. You need to survive long enough to get that Yi or Shadow Lux. I see people going for it and failing all the time. Especially when Yi does not appear on the 4-4 carousel, or somebody not even playing Shadows grabs him because they know what can happen if they let a Shadow player get him.
And even if you manage it, it's not an automatic win either. Other players may be just too far ahead at that point.
It's just one of the current meta builds. But I wouldn't exaggerate as far as claiming that it wins (almost) every time.