The thing about constructive criticism, when it comes to something like programming language design, is that its usefulness varies inversely with the importance of its aim.

Core features of the language are highly important (obviously) but criticizing them is not very useful since they're highly unlikely to change. Thus, only the most peripheral and superficial parts (as well as new additions) are subject to valid, constructive criticism.

People who have criticism for (or are otherwise skeptical of) the core parts of Rust are not being constructive, then, and so these fights can break out.