BismIllah wa as salaamu alaikum wa Rahmat Allah wa Barakatuh.
I have pondered the issues that are generally faced in polygyny for years. In fact, I came to a conclusion that it was imperative that everyone be on the same page, have a level of taqwa and emaan combined with the desire to live solely for the pleasure of Allah, to be able to make it a success. I still hold to that, because without it, fitnah abounds. However, something much bigger than that is really at the root of polygynous success. It's all about the man.
The pious man, truly connected to his Rabb, will select his wife based on criteria and Divine guidance. The pious man will fear Allah in his dealings and strive to correct his failings and be a good husband. The pious man will marry for the best of reasons. So, before anything else, the husband must be a good, pious man.
Because polygyny is a magnifier. The wife knows her husbands faults, shortcomings, weaknesses. She knows and it is her duty to cover those faults and to encourage improvement. However, what happens when another wife is brought on the scene is that the same things the first wife knows about, the second one learns about too. So if the husband lacks patience or is a poor communicator, he has two women telling him about it or reacting to him because of it. Whatever he has weaknesses in that cause problems or difficulties in a relationship, become two sets of difficulties...or three...or four. The problems do not disappear by taking another wife, they are magnified and duplicated. I have witnessed this with my own eyes. Even if the first wife has remained silent about the husband's shortcomings, often the newness makes the second or third mention them. Usually, however, they are issues that have already arisen in the first marriage and are simply repeating themselves in the subsequent ones. So from every angle, he will hear that he is not fair, or not admonishing correctly, or not communicating well, or not having compassion. It puts him under the microscope and magnifies his deficiencies. So, at the end of the day, it is the husband who it all rests on. Nearly every problem that has arisen in polygyny was caused by improper practise and deficiencies in deen. Those issues hinge on the husband foremost, and spiral out. Often men cannot handle such brutal exposure, but if they are truly striving for Jennah, they will realize that it is an opportunity to improve and correct themselves.
May Allah, azza wa jal, increase the emaan and taqwa of us all and keep us firmly on His Perfect Siraat, ever striving for His pleasure and living according to the Quran and Sunnah - ameen.