We tried this for the first time last night. Loving spicy foods, but unsure of the Berbere strength, we compromised on the suggestion and used 4 Tbsp of the Penzy's brand Berbere. It was just right (meaning - very potent, nose-running, but not tongue-lacerating). For most people this level would be considered "very spicy", so know your limits.
We had to put some salt in each bowl to really bring the flavor out, but that little bit did the trick.
We also scaled back the chicken to 1.75 lb to focus more on the veggie content, but that's the great thing about stews - you can tweak them until your heart's content.
The mixture of flavors is superb - the cardamom, ginger, and cinnamon bring sweetness to complexly balance with the cayenne heat.