Mind you, that suggests that the sentence is at least half-true even if "much more complex" is a big overstatement, since Rust, "modern" C++ and the later evolutions of C# are all relatively recent. (What would have compared to Ada in complexity back in the day? Common Lisp, Algol 68?)

As a matter of general interest, what features or elements of Ada make it particularly hard to compile, or compile well? (And are there parts which look like they might be difficult to manage but aren't?)