It looks like Framestore will be doing the visual effects, as spotted by /u/Wortasyy. The source is IMDb, and I'm well aware of how unreliable IMDb is pre-release of a show – but it has been consistently accurate when it comes to The Witcher.

Visual effects supervisor Julian Parry followed them after Platige, so that's more evidence.

They have an impressive CV, working on Harry Potter, Guardians of the Galaxy, Dr Strange, the new Wonder Woman, etc. That doesn't guarantee anything, of course – and the only common shows really linking many of the confirmed visual effects ppl working on The Witcher is a few on The Terror and a Nat Geo series about Mars. But, combined with a Netflix budget, I do think it shows the potential is there.

Would love anyone with a little more expertise in the area to chip in here...