Complex sentence: Establishing the names of Netherlandish masters and attributing specific works has been difficult; the attributions established today are the result of decades of painstaking research by the early 20th-century art historians.
Split into simpler sentences: Establishing the names of Netherlandish masters and attributing specific works has been challenging. Well-established attributions today are the result of decades of painstaking research by the early 20th-century art historians.