I agree that the feeling of immersion is at stake here (I feel this is why it was so off-putting to see Ed Sheeran in GoT scene), but I think this actually depends on the actor. There's so many that lack the talent or the will to mix it up, and consequently just playing themselves in roles. Chris Evans, Megan Fox, Jennifer Aniston, Jason Statham are some examples that come to mind.

Then there's others, like Daniel Day Lewis, Di Caprio, Ralph Fiennes, etc. who's roles comes across as being fiercely distinct and yet still utterly convincing.

This is a total paraphrase and possibly a mis-creditation, but I'm pretty sure Ian McKellen said something along the lines of "When you watch good actors, you think, they're a good actor. When you watch great actors, you actually believe they're a real person."

Christopher Lee is another one, always playing the villain, always playing it good and never just being Christopher Lee.

I think it’s not necessarily bad in it’s own, but no known actors + no known show runner + studio without a track record of success + IP without involvement of its creator and Vague primary sources(ie no specific written narrative to follow) seems like a risky combination.

Nobody becomes known if they don’t have their first time. I think it is not bad at all.

For me, it’s not that there are any unknown actors, it’s that every single one is unknown.

I like seeing “fresh” actors because they are unknown. LOTR had many unknown actors and lots who later went on to notoriety (eg, Karl Urban). But they also had some known.

My apprehension isn’t that there are any unknown, but that all are unknown. And I think that’s a bad thing for a $400M project.

Kind of like startups. I’ve met some great people who were working for their first company ever. I’ve also worked with some terrible people who didn’t work out. To get small funding this happens all the time. But to get $465M in funding with a leadership team that’s entirely new is unusual.

It's not necessarily bad if you have someone amazing in charge of casting, but it's really hard to build a cast out of people who nobody has heard of who it turns out are actually incredible talents. Known actors are a known quantity in that regard, which is why it's safer to go for them. Normally you'd expect there to be some on a big budget project that isn't taking risks around quality.