Preheat the oven to 400°F. Toss the cubed sweet potatoes with olive oil, cumin, chili powder, and salt on a baking sheet. Roast for 25 minutes until tender and lightly browned.
Warm the black beans in a skillet over medium heat for 5 minutes. Stir occasionally and add a pinch of salt if needed.
Combine the shredded cabbage and carrot in a bowl. Add the juice of one lime and a pinch of salt, then toss well to coat.
Warm the corn tortillas in a dry skillet or directly over a low flame until soft and pliable.
Assemble each taco by layering roasted sweet potatoes and black beans on a tortilla. Top with cabbage slaw, avocado slices, and a squeeze of remaining lime. Finish with fresh cilantro.