Complex sentence: His military success against the Ottomans was never extremely successful, as his military abilities fell short of those of Skanderbeg, and also he was unable to unite the country in the way that Skanderbeg had.
Split into simpler sentences: His military success against the Ottomans was never extremely successful, as his military abilities fell short of those of Skanderbeg. He also lacked the ability to unite the country in the way that Skanderbeg had.