John Carney’s latest promises the same laughs and sobs as his previous films, but its oversimplification of social issues is jarring — John Carney is a significantly underrated director. Though Begin Again, his most popular (and only American) film, lacks the flavouring that made both Once and Sing Street wonderful, personal indie darlings, Carney is consistently impressive. His work, always focused on music and the bonds that it can create between complex…