I did see some "Digital Artifacts" on the screen while watching Rogue One, but I think the artists did a splendid job of recreating Grand Moff Tarkin, originally portrayed by Peter Cushing.
That said, I would not have been disappointed if they simply allowed Guy Henry to play the live action part. Audiences want to be entertained and are willing to be a part of the fantasy. That buy-in is enough, IMHO, to overlook a different face in the role of a character.
Tarkin gets a cameo in Ep. III, Revenge of the Sith, and was played by Christopher Eccleston in prosthetic makeup.
the filmmakers weren’t sure how the CG effect of Tarkin was going to look and, because of that, alternative solutions were planned. “We did talk about Tarkin participating in conversations via hologram, or transferring that dialogue to other characters,” said John Knoll, a producer on Rogue One who is also the CCO of Industrial Light and Magic.
To complete the effect, actor Guy Henry was on set dressed as Tarkin, with full performance capture rigs on his head. “We’re transforming the actor’s appearance to look like another character, but just using digital technology,” Knoll explained. That data was sent to ILM, where they did their best to capture Cushing’s nuance. However, they quickly realized that was no simple task—especially after the realization that the lighting in A New Hope was different than Rogue One. Both things upped the challenge of recreating Cushing/Tarkin considerably, but the team eventually felt like they hit a strong balance. It’s “a super high-tech and labor-intensive version of doing make-up,” Knoll said.