the wikipedia article on this subject is very good. http://en.wikipedia.org/wiki/Synthesis_anarchism
There it is shown why platformism was rejected by the most important personalities in that time within the anarchist movement worldwide incluiding Errico Malatesta, Luigi Fabbri, Camillo Berneri, Max Nettlau, Alexander Berkman, Emma Goldman and Gregori Maximoff.
In the spanish establishment of the FAI (Iberian Anarchist Federation) synthesis anarchism was the model adopted and the Dielo Truda platform rejected because they saw it was too marxist and sectarian. Before and during the early days of Mussolini´s Fascist regime, the Unione Anarchica Italiana was a large national organization based on more or less synthesis principles and it was the main antecedent of the contemporary Federazione Anarchica Italiana. Also before and during the insurrection led by Nestor Makhno, there existed in Ukraine the NABAT Confederation of Anarchist Organizations (http://en.wikipedia.org/wiki/Nabat
). The principles of Nabat were written by synthesis anarchist theorist and fighter in the Makhnovist army Voline(http://en.wikipedia.org/wiki/Volin
Today synthesis anarchism manifests itself in the International of Anarchist Federations (IAF)(http://www.iaf-ifa.org/
) which includes the biggest anarchist organizations worldwide. There are the large national synthesis federations such as Federation anarchiste (composed of about 60 local groups nationwide) from France and Belgium, the Federazione Anarchica Italiana (about 30 local groups nationwide) (http://federazioneanarchica.org/
) and the post-Franco refundation of the Iberian Spanish Federation (http://www.nodo50.org/fai-ifa/
In all of the countries where there exist a IAF national synthesis federation, that is the main anarchist organization. Other countries where there are IAF federations are Argentina, IK and Ireland (the platformist oriented Anarchist Federation), Bulgaria, Germany and Siwtzerland, Belarus, and Czech republic and Slovakia.
In the case of France in the 1940s the Federation Anarchiste experienced an strange episode when a platformist faction led by George Fontenis rose up to the general secretariat and achieved imposing a rule of majority vote. Through this medium it expelled those opposing turning the FA into a platformist organization. Shortly later the FA changed its name into the Anarcho Communist Fedetation (FAC). This new organization didn´t last for long and those who were expelled by Fontenis and his group reorganized a new synthesist Federation Anarchiste which is the one that lasts until today in which important personalities who wrote the foundational principles included the non-platformist anarcho-communist Maurice Joyeux and the individualist anarchist freethinking and naturist Charles Auguste Bontemps (http://en.wikipedia.org/wiki/Charles-Auguste_Bontemps
Platformists complain that synthesis anarchism Federations have included individualist anarchists because of their adherence to anarchism without adjectives. Individualist anarchism as it has been understood in Europe (see http://en.wikipedia.org/wiki/Individualist_anarchism_in_Europe
) is an eclectic tendency which has included free love actvism, anarcho-naturism nudist ecologism (see http://en.wikipedia.org/wiki/Anarcho-naturism),
freethinking and atheist activism (see http://en.wikipedia.org/wiki/Freethought),
alternative education, intentional community experimentation among other things which platformists are not interested in.