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

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.