Flannel quilts up beautifully. I’ve made many, many flannel quilts over the years, prewashed, not prewashed, some fabrics prewashed others not in the same quilt. They are all beautiful. Many things play into the shrinkage issue. Batting used, quality of the flannel, amount ( density) of the quilting, amount / size of piecing. Everything factors in. If the top is pieced it needs to be quilted and bound before laundering.