Zig's unique power and killer feature isn't having comptime; it's having little else. That's a feature C++ or Rust or D or Nim simply can never, ever have, and it's an extremely important feature, especially in low-level programming. You can do in C++ anything you can do in Zig; but you can't do those things in a simple language you can quickly learn and fully grasp.
Take this with a grain of salt but from the little examples I've seen it looks like a nightmare for any type of large application. I would much rather have increased power in the type system rather than having arbitrary code run and fail builds in an ad-hoc fashion.
It isn't "arbitrary code." It is strictly less arbitrary and ad-hoc than Rust's macros. You can think of it more as a programmable type system, although that, too, is not very precise. As to maintenance of large codebases, it is far too early to tell, of course, but note that no low-level programming language has a great record on that front. I think it is because components in such languages are much more sensitive to the implementation details of others (i.e. all those languages have very low abstraction, i.e. the ability to hide and isolate internal implementation details), but low-level programmers know this comes with the territory, and is part of the price you pay for a high-level of control over low-level details.
