Darwin Nunez still needs time to adapt to Liverpool's game. Even Liverpool don't play well as a team these days so it would be unfair to blame things on Nunez. He was a wonderful striker while he was playing for Benfica. He was like a goal machine I mean. However the Premier League is a completely different environment. Teams here play with very different strategies and the tempo is always high here. Liverpool are one of the teams that love to play fast. After some time Nunez would start to adapt hopefully otherwise of course his end might be like Werner for Chelsea.
Haaland is a way more skilled player than Nunez. He was already playing for Dortmund that liked to play really fast so he didn't have any problem on adapting also. He is most probably going to have a much higher level of performance than Nunez during the whole season. But Liverpool wouldn't care about this comparison. They'd only like to see Nunez meeting the expectations after spending a huge amount of money for him. For now, Firmino seems to be better than him in the team.