FDR's call for the "unconditional surrender by Germany, Italy, and Japan" was certainly controversial, but mostly within the U.S. State Department, and later with the U.S. government as Truman and his advisers felt that this somewhat tied their hands in regard to Japan.

I am not aware of any views from Stalin, Chiang Kai-Shek, or any of the other allies, but Churchill's main concern was the inclusion of Italy (as well as satellite Axis powers such as Hungary and Romania) whom he felt could be convinced to surrender under favorable terms. He wrote in his memoirs the following about when it was first discussed:

There is a school of thought, both in England and America, which argues that the phrase prolonged the war and played into the dictators' hands by driving their peoples and armies to desperation. I do not myself agree with this, for reasons which the course of this narrative will show. Nevertheless, as my own memory has proved defective on some points it will be well to state the facts as my records reveal them. Elliott Roosevelt asserts in his book that the words were used by the President at one of our dinners. I am reported by him to have "thought, frowned, thought, finally grinned, and at length announced 'Perfect.', and also that the nightcap toast proposed by Mr. Churchill that evening was 'Unconditional surrender.'

In his notes he refers to the unconditional surrender of Germany and Japan, but not Italy.

US Secretary of State, Cordell Hull, has the following to say on the subject in his memoirs:

The principle of unconditional surrender overshadowed our policy toward the Axis and their satellites and our planning for their future

Originally this principle had not formed part of the State Department's thinking. We were as much surprised as Mr. Churchill when, for the first time, the President, in the Prime Minister's presence, stated it suddenly to a press conference during the Casablanca conference in January, 1943. I was told the Prime Minister was dumbfounded.

Basically, I was opposed to the principle for two reasons, as were many of my associates. One was that it might prolong the war by solidifying Axis resistance into one of desperation. The people of the Axis countries, by believing they had nothing to look forward to but unconditional surrender to the will of their conquerors, might go on fighting long after calmer judgement had convinced them that their fight was hopeless.

Dean Acheson in his memoirs stated that the State Department's policies during the war were generally more "percipient" than Roosevelt's call for "unconditional surrender." Undersecretary Joseph Grew felt that guaranteeing the status of the Emperor could hasten the surrender of Japan, and they should place every tool possible in the hands of the Japanese peace faction. Truman and Byrnes were both concerned about how backing off of FDR's stated position would play in Congress and the public, although the Potsdam Declaration used a noticeable softening of the diplomatic language, calling for the "unconditional surrender of Japanese armed forces." The term "unconditional surrender" was certainly a hurdle that had to be overcome in the minds of the Japanese government in order to end the war.