Al Nassr has Ronaldo Al Hilal has Alexander Mitrovic and Neymar and Al Ittihad has Karim Benzema so bringing in Ronaldo alone is not enough to make Al Nassr win the title
They are three biggest clubs in Saudi Arabia and in this season, they are most competitive clubs to compete for Saudi Professional League title too. I really disagree to say count Neymar in this example for attacking power of three competitors. Neymar only played 5 matches for Al Hilal and he just failed a medical test, and his return time is very unknown now. With his very limited contribution for Al Hilal, I don't count him in this example.
Al Rayyan was not easy at all and just like the other Arabian teams they had many expensive players in this team who could change the result in this match and even against Al Nassr, Al Rayyan was not a weak team and that's why I can say Pioli had a good start in this team because of the hard match they had against Al Rayyan.
No opponent is easy to beat and if you are not strong enough and don't play carefully, you will lose to your opponent, even they are not too strong or a weak team. Because in football, one mistake can be punished by your opponent with a goal, and it's simple if you can not equalize it, you lose the match.
Now after hiring Pioli maybe Al Nassr can have more hope to even beat Al Hilal finally after losing against them. The situation can be better for Al Nassr when we see Ronaldo is also in good form again.
Pioli with his first matches for Al Nassr helped them to win but his main task will be improvement on Al Nassr's defense system. It needs to be tightened and improved a lot to make Al Nassr more competitive. You can win a match if you don't get any conceded goal and manage to score only one goal, but you can lose a match even you score 3 or 4 goals but you get more conceded goals.