Actually, I would have thought it was relatively easy to compare as, firstly, we are comparing violent crime, definitions of which are similar everywhere, particularly so when comparing between two common-law jurisdictions. I expect there is a comparison somewhere. Do you have one? If you don't, don't make the claim.
As for South Africa, gun ownership and crime is so poorly recorded that their statistics can't be relied on. The police routinely don't investigate crimes because they are under-resourced, disorganised and bribable. For historical reasons, there is a culture of suspicion of the government, and therefore gun control is routinely circumvented. And finally, there is corruption in the government too, so a 21% drop in (what exactly?) could simply be the government instructing their statisticians to release a good news story.
You've answered your own question right there...
this is why it's not easy to compare like for like, as some cultures will not report it, or if it is reported it'll not be recorded.
Since you ask for examples of issues with differences:
www.unodc.org/unodc/en/data-and-analysis/Compiling-and-comparing-International-Crime-Statistics.html
en.wikipedia.org/wiki/Violent_crime#cite_note-eurosb2010-1
A good example is that in the UK sex offences count as violent crimes, in New Zealand they do not.
Most of Europe counts minor violence against a person as assault, whilst France does not.