Rust

Rust is one of the harder languages for beginners to learn because of its borrow checker and strict ownership model, but it shouldn't take more than a month or two for a competent senior to pick up.

It's going to be deeply unpleasant and seem like a problem if:

• You're writing dangerously bad C or C++ code already.
• You've only ever used Python or JavaScript.
• You try to shoehorn OOP and inheritance into it (Rust idioms are composition and functional programming).
• You refuse to use/learn pattern matching.
• You're a pedant about "pretty" syntax.

If someone is at a senior level and any of those apply, they probably shouldn't be at a senior level, though.