One thing about Suns is that they have way too much offense to their very little defense. CP3 is not the defender he used to be, Booker is not a great defender, and Ayton is a good rebounder but couldn't say he was a rim protector, not a feared one at least, and Durant is great at defense but he will be pretty tired on scoring constantly on offense, so he can't really put all his efforts on defense.
This concludes that they won't really have a great defense, could be good, just good, but not great and they may have some trouble on playoffs. They can play a lot better on regular season because people don't defend that hard, but playoffs could be a trouble.
I believe the Suns will be able to make up for their weak defense with their offense, especially now that Durant has joined the team.
But a determining factor is that, in the playoffs, the games are more competitive and depending on the physical conditioning of a player, he may be injured during the playoffs. Unfortunately CP3 is one such example.
If the Suns really stand out in the playoffs and manage to reach a WCF, he will need to rotate and always be changing CP3, so he doesn't get injured, but an extremely positive point that CP3 has is his experience, I have no doubt that this could be an important thing for the Suns, especially in the playoffs, which for the most part separates the men from the children.
I would love for CP3 to get his championship ring before he retires, he deserves it, he had an excellent career in the NBA, and unfortunately, I have no doubt that he will retire soon.