Take Ukraine, for example. They do not allow dual citizenship, but at the same time they do not revoke Ukrainian citizenship automatically. It sounds illogical, but it is true

You are still considered Ukrainian citizen, but the consulate will not do anything for you if they know that you are a US citizen. For instance, to get a new passport from consulate, they need a copy of your GC. If you are an honest person (such as I am

), you will not send them a copy of GC because you are not an LPR anymore. But then you will not get the passport and will not be able to travel to Ukraine... unless you travel on a US passport... which is illegal because you are still a citizen of Ukraine... Hmm... Are you with me still?
If I want to renounce my Ukrainian citizenship, I will have to collect dozens of documents (some of which can only be obtained in Ukraine in person), spend $$$, travel to the district consulate and then wait for up to 2 years. I personally gave up during the stage of collecting documents because in order to get some of them, I would have to register with Ukrainian military service, obtain Ukrainian tax number and renew my internal Ukrainian passport. To do the above tasks, I would have to travel to Ukraine and spend several months there. I just can't spend that much time.