@Auspice
Respectfully, those are all fairly big actors and actresses.  Let's just look at the Skekses:
Andy Samberg: The boy from The Lonely Island?  He'll be fine.
Jason Isaacs: I'll take Captain Lorca from Star Trek: Discovery any day.
Warrick Brownlow-Pike: He's a puppeteer for other Henson productions.
Ralph Ineson:  Brits are, by and large, trained for this sort of performance.  Also, he's the Demonicist from Star vs. The Forces of Evil.
Olafur Darri Olafson: Voice credits include The BFG and How to Train Your Dragon: The Hidden World.
Sigourney Weaver:  Plenty of voice work.
Benedict Wong: The voice of Alex Yu from Prey can do no wrong.  Also, he's Wong from Doctor Strange.
Simon Pegg: Another actor whose talent I cannot question.
Toby Jones: Hail Hydra.  Also, Owl from Christopher Robin.
Keegan Michael-Key:  More voice credits than most.
Harvey Fierstein: The one and only.
Mark Hamill:  Duh.
I have no worries for the voices; I'm a little hesitant for the writing.  The Dark Crystal was wonderful to look at, but painful to watch at times.