I'm planning to play competitively at stores, so I'll be playing by the book there.
But I also want to play with a group of friends who love 40k but are more into the fluff of games than being competitive. I've already showed them the alignment wheel, and they are completely turned off to the game because of it. They are only willing to play the game if we play mono-faction decks.
I'm pretty sure that there will be some factions that are weaker by themselves. I'm wondering if there are any factions that are completely overpowered in a mono-faction environment. I'm also wondering if tyranids (who cannot take allies) will be overpowered in a mono-faction environment because they have been built to not require allies.
Also, please understand that I'm not posting this thread to ask what you think of the alignment wheel. That has been discussed extensively already. Please don't try to convince me of your opinion of the alignment wheel, there are many threads for you to vent about that. I'm also not asking for your opinion regarding whether or not mono-faction decks follow the fluff more closely than decks with allies.
I'm simply asking about power levels.