Hacker News Reader

Just saying that these "feel like toys" is a very low-effort comment. Header-only libraries exist even if uncommon and they work fine when written properly.

If you actually use a header-only library and it fails to link properly or be used in a project with multiple compilation units, then that would be a bug in the header-only library worth discussing and fixing. That would be a good-effort and interesting comment.

sqlite has amalgamated source too. It's used EVERYWHERE. This opinion doesn't seem like it's based on anything valid or real.

Note you get some opportunities for better compiler optimizations when the entire compilation unit is the entire project. In fact, sqlite claims the code runs 5-10% faster when built as an amalgamation (https://www.sqlite.org/amalgamation.html)

Unless it contains C++ templates, it's fairly trivial to compile a "header-only" library and use it as normal, even in unusual cases where the author has made no effort to support that.