Predictions and implementation are only as good as the programming behind them. I can without a doubt state that the UFMC has fantastic representation of VNAV provided that the config file you're using is well coded. The developer of the UFMC provides some preset ones for you when purchased. Bear in mind that the UFMC does NOT use X-Plane's generic VNAV implementation. Instead, it uses custom logic (just like PMDG) to properly calculate things. In fact, the UFMC pretty much overrides the entirety of X-Plane's autopilot with it's own, creating a much more true to life experience. At this moment you are taking something PMDG says at face value. There are people justifying experience here who HAVE flown PMDG products plenty, and are vouching for the integrity of the UFMC and the experience it provides. You can take it at face value as well, or you can purchase it and see for yourself!