Let’s be candid: Radu was unpopular way before anyone could have dreamed that King Michael would make Nicholas a prince and a dynast.
He was not trusted when he married Margarita because some Romanians who knew the Royal Family were already aware of his and his family’s political associations.
He was not popular in his former role as Special for the Romanian Government for his tendency to overshadow his wife, who is the real royal.
He is not popular because he fluffed up his CV and rose through the ranks of the Romanian military in a fashion that has never been properly explained.
He is not popular because he has cultivated, by his own actions, the reputation as the éminence grise of the Royal Family, holding sway over not only his wife but her parents as they aged, and then over her sisters and nephews and nieces.
People on this board might have forgotten or been unaware that King Michael, Queen Anne and their daughters were involved in an insular cult called the Moral Re-Armament Movement while living in exile in Switzerland. Anne wrote about this in her memoirs, which were published in English. She admitted that she became bitter against the world for what had happened to her husband, and that Michael was often overcome (understandably) by moroseness due to losing his country. They did not have a social life, so the MRA helped with that. They only saw close relatives like the Greeks. The family was not well off. Michael was not made to be a businessman - he was raised to be a constitutional monarch. Only Margarita went to university, and the fees for the boarding schools for the four younger princesses were all paid for by Romanian monarchists in exile. The daughters were not taught Romanian, aside from the Lord’s Prayer, which their father used to recite every night. Essentially, the family was shut off from the happenings of the outside world, and they were vulnerable, and then the floodgates opened in 1989. The family were sitting ducks for a strong personality, which they surely got when Radu married Margarita. The Queen was a force to be reckoned with, too, but in the last several years of her life she was ill with lung cancer and what seems to have been dementia. Michael’s health was not great either. In addition to the cancers, he also had issues with his memory, which may have been dementia. The Royal Household was wise not to issue actual medical bulletins provided to them by doctors. This is why there are those who feel that Margarita and Radu issued the August 2015 decision in his name. I do not believe an actual copy of it has been seen bearing the king’s signature, but forgive me if I am wrong on that.
They’ve done the best they could, but they were and are by no means the model of a functional family.