Actually Karim Benzema and Neymar are first season with Saudi Pro League teams and they need time for adapting about weather where difference condition between Europe and Saudi Arabia, its they can adapting well not close opportunity to make compete with Cristiano Ronaldo in top scorer standings position. Its not really bad with Karim Benzema performance in this season, from 6 matches in domestic league he has contribution with 3 goals and 2 assists. Just waiting moment only for Karim Benzema lead top scorer from Cristiano Ronaldo have adapted well with Saudi Arabia weather after second season played for Al Nassr. Not depending with Karim Benzema and Neymar more younger than Cristiano Ronaldo but depend on how long with Ronaldo have played in Saudi Pro League than Karim Benzema and Neymar as their first season with Saudi Arabia club.
Both players are top class and their performance this season is not really bad so far, like you said Benzema has played in 6 matches for Al-ittihad since his arrival and has scored 3 goals and 2 assists which is not very poor however Neymar as well has a good start since his arrival he's been played 3 times and produced 2 assists, though they might not come close to Ronaldo's goal scoring ability of which they never did even while playing for the top European teams they played for but they're good in their own ways.
Well Ronaldo is expected to adapt in his team more than they do for there's cause he's been at Al-Nasr since last season whereas Neymar and Benzema just got in this summer, and still have to blend in well to start performing like Ronaldo and i believe we're yet to see their best in the league cause they're both world class and proven players in their previous clubs.