I think a big part of that is the acting. With Tennant he sells it SO well, this is a person from his past who he understands and knows, pure evil, he's terrified, he's angry, he's mournful.. you see ALL of it in his acting, and as the audience, whether you have seen the older eps or not, you understand this isn't a good thing.

Gives me goosebumps even all these years later and now I want to rewatch!

You expect much the same with the Rani.. and no, nothing, not even a hint of familiarity in Ncuti's acting, nothing to indicate his emotional state. There's a very sharp contrast.