If you know something to be fake, your brain generates any number of reasons that it looks fake, even if they aren’t true or not noticeable to the average viewer.
I thought some of the Tarkin stuff was seamless, despite knowing it to be CG. If motion capture got good enough to capture the micromovements of facial muscles, I would feel a lot better about an actor using a ‘digital likeness’ of another actor (provided the actor or their estate were in agreement). In this case, the actor wearing the digital likeness would be translating much of their performance to the screen, even more than an actor with layers of prosthetics on their face.
I think the most noticeable thing is that when he talks, the muscles around his mount and jaw move in the same exact way every time making him look somewhat mechanical.
I’m still very impressed by the technical achievement and it will get better and there will be chances to improve on those scenes in special editions.