Guerilla's tech and artistry has improved a LOT since the last Killzone game. I haven't seen enough of HZD yet, but it sure looks impressive.
As for Tarkin, the recent FXGuide article has revealed a lot about what seems to be the issue:
https://www.fxguide.com/featured/part-3-rogue-one-digital-humans/
It seems, to me at least, that it comes down to the photography.
New Hope was shot with very different cameras and thus with very different lighting and make-up. Tarkin looks and feels different mostly because even the actor himself would be the same in these conditions. I'm guessing that ILM realized this way too far into production, past the time of shooting, and so they had the no-win choice of either to post-process all the live plates to fit SWIV, or try to fit their CG Tarkin into a very different look.
If you look at the CG version of the stand-in actor, it looks completely convincing even in the test shots (which had no post work at all). I'm quite sure that ILM would've been able to do a completely convincing CG Tarkin for New Hope as well - but the difference in the lighting and such has worked against them and it was too much for the fans.