In general, metaprogramming capabilities(macros, templates, some forms of late binding) create brittle, high-friction abstractions when applied unjudiciously. They're like writing a customized compiler, except the resulting error messages are all awful or nonexistent and the code can't be debugged as easily.

Thus, even in languages where metaprogramming is a prominent feature(e.g. most Lisps) the culture tends to evolve towards writing in a plain style and sparingly using these abilities because they have so much footgun power.

Now, it's possible that Rust's method of extension doesn't have the same capability to cause harm as something like macros, and is more along the lines of generic types, which have a narrower scope and are more in line with everyday needs. But everyone who's seen it happen is justifiably wary about a proclamation of "this time is different".