[T]he burden of both estate taxes and inheritance taxes is borne predominantly by heirs.
From this pre-print of an academic article on the subject (which should be read with the understanding that changes may be made before the article is finalized to reflect comments and additional research).
The fact that death taxes are borne by heirs is important politically, because this fits the notion that these taxes function as a tax in lieu of an income tax on the inherited wealth of heirs, which is the primary political justification for these taxes.
Inheritance taxes are much more common than estate taxes, internationally.
While both inheritance and estate taxes primarily impact heirs (as opposed, for example, to decedents), the burden on particular heirs varies considerably between inheritance and estate taxes. This flows mostly from variations in the number of beneficiaries to whom bequests are left, which in turn is impacted by the number of children involved and the number of non-child beneficiaries in an estate (the study excluded bequests to charities and spouses for purposes of the comparison, assuming that both would be tax free in either system).
[A]bout 30% of inheritances come from estates that have no child beneficiaries, and the plurality of inheritance flows from such estates go to five or more non-child beneficiaries. In addition, about one third of estates have no child or non-child heirs, although they may have spousal or charitable beneficiaries . . . . the number of donor children is actually a very poor proxy for the donor’s total number of heirs. As the number of children rises, the number of non-child beneficiaries declines.
The degree to which the stereotype that people leave their estates overwhelmingly and entirely to their children, to the extent that they do not leave inheritances to charities and spouses, is far less accurate than one would expect. Indeed, despite having spent more than a decade preparing estate plans, I wouldn't have guessed just how large a share of inheritances don't go to children, spouses or charities.
Part of this may have to do with wealthy individuals recognizing that their children, who are often in the primes of their own economically privileged lives at the time an inheritance would be received, often don't have a compelling need to receive an inheritance. It also isn't entirely clear how bequests to trusts, which are common in large estates, are categorized.
Presumably, an inheritance tax would tend to favor collateral heirs who often receive fairly modest bequests, while burdening children, particularly in small families, who often receive fairly large bequests. This explains, in part, why countries with inheritance taxes frequently impose higher tax rates on inheritances from collateral relatives than inheritances from parents.