There's always going to be a "whatever" factor for any country. You try and make your immigration system as robust as possible, but there will always be a few who manage to slip in/ or domestic home grown ones.It's the "whatever" bit that worries us!
You should see the massive binder I have now with all the documents I had to provide for my OZ (and previously the UK) visa, with all police records and and a bunch of stuff all the way from 18, in all the countries I lived in. It ain't exactly easy to get in, unless say you're an EU citizen going into to the UK. Or a Kiwi going into Aus or vice a versa.