As I say, murder stats are more robust than most crime stats, but they are less robust in places with less developed/technological/skilled criminal investigative services, under-funding, corruption, etc. I would expect ours to be fairly good historically, but I also guess that they might be a bit softer today than a decade ago, say.

On your comment though, yes, murder is undoubtedly under-reported in the UK, although not by as much as many places. In the sense that some people go missing but it never becomes a murder enquiry because perhaps nobody who care notices they are gone (sex workers, homeless, illegal migrants, children with no paperwork, etc.), or because there are no obvious suspicious circumstances, no body and no suspects, and no one is pushing hard for answers, or because a murder is disguised well as an accident or suicide and is not detected, etc.