The main aim of signing Ronaldo for Juventus was clearly to win the Champions league, which they didn’t achieve. I see this as the main reason why people claim Ronaldo wasn’t a good signing for Juventus. As well as him being one of the main reasons for Juventus’ economic issues.
Juventus were already the dominant force in Italian football before he joined (6 titles in a row) so it’s hard to say he actually improved them which does raise questions about the value for money they got from him. You have to think that they would have been able to maintain that dominance for far longer by signing a couple of very good younger players for £50m each rather than one 35yr old for 100m