That's not right to say Neymar was useful because he attracted attention to the Saudi League. I think Neymar was a failure for Al Hilal and if they wanted to attract some attention they had Ronaldo in Al Nassr and Ronaldo was famous enough. Even this player was the reason for many other famous players from the top European teams to join this league in Saudi Arabia.
But hiring Neymar didn't damage Al Hilal and they are still the first team of the Saudi League.
It can be a failure on field if Neymar will continue to get injuries after he returns from the latest injury. It's still time for Neymar to perform and contribute for Al Hilal as his current contract with them is two years, not one year.
Comparing Cristiano Ronaldo, Karim Benzema and Neymar, it seems the Brazilian player is a most sensitive player with injuries even he is a youngest player among three biggest star players in Saudi Professional League. I won't expect Neymar will improve his body too much as ageing will continue to be his biggest issue and with time, it only become worse, not better.
Nobody really understood what the goal of Al-Hilal was when they signed Neymar because he was still injured when they signed the contract. They must have known about all the risks and yet they decided to buy him. For PSG the deal was amazing, but to be honest who knows what that deal was really about. Everyone behind these clubs from PSG and Saudi Arabia are businessmen who might have connections elsewhere, too. I doubt that they were hoping Neymar would be as healthy as never before and play a whole season without problems. This illusion burst quickly like a soap bubble.
I think Al Hilal knew about Neymar's medical record and risk of signing him because he is very sensitive with injuries. They have too much money and got support from PIF but I believe that they actually did some analysis about risk, cost and benefit before signing the Brazilian player and giving him too good offer. They maybe wanted to use Neymar as a magnet to attract more star players to their club and also it's a part of Saudi Arabia and PIF's plans to develop their national league.