They ARE having problems.
So... You linked to an article that didn't actually name any of the nations at risk? That's handy
I found the report. I have not read it in detail; however, it identifies and focuses on 19 nations that are "severely vulnerable," which includes countries like Angola, Gabon, Zambia, Venezuela, Lebanon, Somalia, Belize, and so on.
Several of these nations have fairly low (official) debt-to-GDP ratios, such as Gabon (58%), Ecuador (49%), Zimbabwe (20%), DR Congo (14%). I.e. the problem for some of these nations isn't that they were profligate with government spending for decades. More likely is that COVID, general instability, violent conflict, recovering from conflicts, and/or previous debt issues put them at risk of defaulting in the next decade. Given that default is a pretty routine occurrence, this is kind of in the "dog bites man" category.
Oddly enough, many nations with the highest debt-to-GDP ratio -- like Japan (237%), Greece (174%), Italy (133%), Singapore (109%), and the US (106%) aren't on the list.
Another oddity?
The report doesn't actually discuss the harm caused by those debts or defaults. I'm certainly not saying that "this means the UN does not believe that default causes harms" -- far from it. It's more that
the report doesn't answer the question of "what harm has federal debt caused for advanced nations like the US, Japan, Italy or Singapore?"
As to Montengro? For many years, China has been offering huge loans for infrastructure projects with many developing nations, with tons of strings attached. Many nations are having problems, because they somehow assumed that China was going to behave like other lending nations or institutions. Nope, China has frequently required that they use Chinese contractors, or demanded control of the project if the nation can't pay. In contrast, loans from other nations or institutions aren't structured this way.
However, it's not clear what the harm is here, other than the highway not being finished, or China exerting undue influence over Montenegro. For example, China's "Belt and Road Initiative" was heavily criticized for using a heavy hand in Sri Lanka; when the Hambantota port (built with Chinese loans and engineers) underperformed, SR couldn't pay back the loans. Sri Lanka decided to sell 80% of the port, which was bought by China Merchants Port (a Hong Kong company partly owned by the Chinese government). Although it initially looked like a raw deal for SR, the port quickly turned around and is now a success, is well positioned for future success, and SR decided in 2014 to build another seaport... with Chinese loans and support. There is a lot of reason to be suspicious of Chinese debt traps, but they may not be anywhere near as bad as often assumed.
Anyway. Needless to say, Montenegro is very different than the US. It's tiny; its tax revenues are much lower; it doesn't have the wherewithal to tell China to sod off; it has exposed itself to a bad deal with China; it has a lot of restrictions on fiscal policy because it's trying to join the EU; there are no strings attached when China buys US debt, and so on.
The list of ways that the nations at risk are vastly different than the US goes on.
If I gave the impression that "no government has, or ever will have a problem, with debt" then I will cheerily be corrected. It should be pretty obvious I don't hold that view, as I've discussed default and examples like Greece, but that's OK. Thus to clarify, my position is:
Federal debt is not a serious problem for the United States, or many other advanced nations, even those with very high debt-to-GDP ratios.
So. Japan has a debt-to-GDP ratio over twice that of the US. Why aren't they on the verge of disaster? Why aren't they at risk? What problems have they had which are
directly and primarily attributable to their high government debt?