Heat the oven to 400° and line a large baking sheet with parchment paper.
Toss the sweet potatoes with the olive oil, chili powder and salt and pepper, then roast in the oven until golden brown—about 25 minutes.
While the sweet potatoes roast, combine the radishes, scallions, apple, lime juice, olive oil, cilantro and salt and pepper in a bowl. Toss to coat, then chill for 30 minutes.
After chilling, season the slaw with additional salt and pepper, if needed.
Assemble each taco with sweet potatoes, slaw and diced avocado. Serve with lime wedges.