Modern archery equipment imparts a great deal more energy into the arrows than yesterday's stuff could . Whenever you increase the energy, you decrease the amount of acceptable tollerance you can have to stay in spec. The smallest imperfections get amplified and can cause big inconsistencies.
One problem with some cheaper carbons is that they are made in such a way that results in different spine values if you bend the shaft different ways. If I buy carbon again, I'll spend at least $100 per dozen.
I currently use aluminums, and shoot plenty tight for hunting out to 40 yds with XX75s. Cheap aluminums are much more forgiving than cheap carbons.