Photorealistic CGI faces are hard. Photorealistic CGI performances are ten times harder, even with really good motion capture. An example from Blade Runner 2049: https://www.youtube.com/watch?v=fV34mT5m0bM
In the side-by-side comparisons, the digital double looks nigh indistinguishable from the original, but translating that into an entirely new performance leaves a lot to be desired. Look at the eyes - human eyes constantly shift and refocus in a dance of micromovements, and you can see the model shifting its eyes in a way that mimics how humans scan another human’s face, but it is still too slow and obvious. Similarly, there is a lack of coordination between the muscles of the face, and that same smooth animated feel. In reality, the muscles of the face are constantly twitching in close coordination to betray the extremely subtle emotions that may not even register to our conscious minds.
All of these problems were apparent in Tarkin and Leia from Rogue One.