Here are my worries:
1. When not including PKs, Ronaldo may not score more than 10 goals in a much tougher and rougher league.
2. He might cause locker room trouble because of playing time and wages.
3. We might have him for 3 (or 4) years paying top wages and be afraid to move on from him. Much like Rooney's last 4 or 5 years.
4. Cavani's minutes will be reduced too much and I think Cavani is the better striker in terms of goals and team play.
5. He is too old to win anymore trophies and the last year is proof with Juve and Portugal.
The only thing Ole could do, I think, is rotate Ronaldo and Cavani in the striker role every match. One match Ronaldo starts and Cavani subs in, second match, Cavani starts and Ronaldo subs in, etc... This way they both keep sharp. The 3 sub rule in the EPL, however, makes this untenable. In addition, Ronaldo will be really upset when he does not play an important CL match.
I think we were expertly played by Ronaldo's agent and City. We were not interested in him until it looked like he might be going to City and the United brass decided they could not let that happen. Now we have an old, aging star who could fade really quickly.
However, I am looking forward to his first introduction at Old Trafford. What a spectacle it will be.