I don't know what you mean by functioning. The US had an estate tax systems. It functions. The issue is whether it serves the purpose for which it was created.
I suspect that European billionaires find it just as trivial to legally avoid estate taxes as they do here.
There seems to be a naive understanding of the manner in which the 'rich' hold their money and wealth. It's not sitting in a nice and tidy 401(k) that is easy to identify and tax. Instead -- if they know what they are doing -- it's a convoluted web of interests in international companies, foundations, and contractual rights. If he wished, for instance, Bill Gates (just to use an example) could easily arrange things so that he 'owned' nothing, yet still lived like a billionaire.
Other countries have functioning inheritance tax systems.
> Consider income tax... consider the massive involvement of the govt required to enforce this tax.
The IRS makes about $5 for every additional $1 it spends on enforcement. It more than pays for itself.