We are trying to balance the game. Balance problems, issues, and bugs, have much greater consequence Solo than they do in Coop. Hence, why Solo players can feel "attacked" by that. But really solo play helps best highlight the problems with the game.
I myself play solo most often, and I experience the frustration. The beta is about figuring those things out, and fixing them, before the game is set in stone.
Overall though, I see the game will be "fun to play alone", but certainly won't be *easy* to play alone. Solo play is intended to be the ultimate challenge.