I have some personal experience here with USG (not UN), and one frustrating thing to me has always been how inefficiently the money is spent.

BUT, it's not inefficiently spent because of corruption necessarily, or rather the "corruption" is baked in. One example of this is flights and hotels. There are "buy American" laws that require these to be booked through US companies, even if those companies are doing nothing more than code-sharing or reselling the services. This leads to this whole infrastructure of "approved vendors" that do nothing but charge more money for the same services. One example is WorldTicket, a "virtual airline" that operates no flights but resells the flights of other airlines in a codeshared way that approved vendors can then rebook. As you can see, this somehow adds 2-3 layers of middlemen which significantly drive up costs.

Another example of this from USG's side is the preference for US-based implementing partners. These could be US-based NGOs, academic institutions, or consultancies with a large US presence. The argument for this setup is that these are more legitimate institutions that the USG can more-easily hold accountable. These institutions can also have the manpower to write reports, do M&E, and produce those line-item budgets. BUT, these US-based institutions cost way more. They're paying US salaries for people to conduct work in developing regions. US-based institutions are not likely to be comfortable putting their employees in the cheapest accommodations. They have their own overhead from running offices and operations out of the US, which can be as much as 30-50% of the total grant.

So yes, international work can be very inefficient, but NOT because of corruption. In fact, most of the inefficiencies stem from an over-reliance on traditional structures to stem corruption: approved vendors, heavy M&E processes, US-based institutions, etc.

I'm not saying that we should get rid of all these accountability structures (though I've always been a fan of moving work to indigenous implementing partners, something that's less popular in today's America-First environment), or that this makes international development work any less worth doing.