Citation needed. I, and probably the majority of the citizens of those countries do not consider the standard test/psych eval/background check/random checks in the future to make sure you're following the rules to be "exceptionally onerous". And i think most non-Americans would agree that adding some friction to a fringe case (owning a personal firearm for protection or fun is not something most people do, even in the US) is worth it if it nearly eliminates blatant misuses of firearms - either making suicides easier and more terminal, enabling easier revenge murders, or making your average school/public place shooting easier.
What would you consider a just middle ground between "onerous requirements" and "everyone can buy any weapon without any requirements but paying for it"?