"pure" aka "mono" decks are fine. Most will do extremely well in tournament play. In fact every month mono-faction decks becomes even more viable. The current trend seems to be that the smaller the collection the more advantageous it is to include other factions when building a competitive deck.
I still think, that unless you have a good reason, going beyond two factions in a deck will give you resourcing problems (unless you're cheating those other factions' cards into play of course).
In fact, to those just getting into the game I recommend that they go as much mono as possible. Its the way the game was meant to be played and thats where most people who play this as a board game will enjoy that structure. However, after you get "bored" and you (or your group) becomes more adept with the game you'll have more fun discovering all of the fun strategies by combining factions.
Though of course, not everyone will find this as much as fun as the guy who "found the good one." ;) hehe
But hey, ultimately... everything is pretty open and viable at this point. The difference between a tier 1 deck and a tier 1.5 deck right now is whether or not it has a way to defend itself against the Large Man + An Offer You Can't Refuse turn 1 win combo.
Tom Capor - email: firstname.lastname@example.org
-'09, '10, '11, '12 Call of Cthulhu LCG World Champion <- Woohoo!
-'12 WoW TCG Realm Qualifier Winner <- Oops, I don't even play this game.
-'12 Call of Cthulhu LCG North American Champion
-'12 Call of Cthulhu LCG Regional Champion
-'11, '12 Call of Cthulhu LCG Championship Warm-Up Winner.
-'10 Call of Cthulhu LCG Highlander Tournament * Conspiracy Tournament Winner.
-'08 Call of Cthulhu CCG World Championship runner up.
-'07 Dungeon'sDragons Miniatures Limited Championship Top 8 Finisher
-'05 Duel Masters North American Champion & Grand Kaijudo Master Duel Winner
-'05 Duel Masters Gencon Regional Qualifirer Winner
-'05 4-time Duel Masters "Tournament of the 5 Civilizations" Winner
-'05 Call of Cthulhu CCG World Championship Top 8 Finisher
-'02-'03 Yu-Gi-Oh! TCG Duelist Kingdom Tournament Season 1 - 3 Top 5 Finisher.
There are some problems with mono decks.
All factions are just not viable in mono decks. Miska makes you draw, but it's still not interesting to draw others miskatonic squishy characters.Same problem with shub and board control. You play monsters/dark youngs/ Migo. Ok. and, then what?
So, yes it is more and more possible to win a casual game with mono faction decks but, for me, not in competitive events.
The second problem with mono decks is their high predictive deckbuild. Cthulhu is not a game with a ton of good and efficient cards. So, when you play a mono deck, your opponent can guess with ease what cards you will play.
Biggest to coc 2011 (27 players)
European champion 2012
I love single faction decks and i use them all the time. Another advantage of a two faction deck is that each faction has a style it leans more towards so they lack in certain things. Having another deck helps to even things out a bit, this being said you can easily compensate by making your deck filled with twice the devious cards your faction has as oppossed to if you had two factions. Try to pick a theme wehn creating a deck that helps a lot.