Betrayal makes sense with some factions. But with the way it's been going in C&C lately, it's gotten REALLY boring.
I can see it with Nod or the GLA. I can see it in certain extreme situations with GDI. I can see it to an extent with the Soviets, what with purges and all.
The Allies should never fight other Allied armies. GDI should only very rarely fight another GDI army (I think we've used up their quota already). USA=no. EU=no. China...probably not.
Anyone else...well, we'll see, but I'll only accept it in a faction that it makes sense in.