From contemporaneous reviews
“The ISCM Festival in London in the first week of June was the first that I have been able to attend from beginning to end, and it has left me wondering just what the purpose of this theoretically estimable body can be. Is it to give young composers and performers a chance of keeping abreast with the best and most significant new music? If so, why was so much poor stuff performed, and why so little by the composers who (rightly or wrongly) are generally recognized as leaders by young musicians throughout the world?
“[I]t would be a good idea if every ISCM festival brought together not only composers but performers from all the countries that support any kind of active musical life, so that really authentic and persuasive performances could be guaranteed, and not mere well-intentioned approximations that convince no one: despite all the organizing committee’s efforts there were not a few of these.
“[U]nless we are to admit publicly that there is something about the present state of world culture that prevents the composition of great music (and presumably this is the last thing the ISCM could afford to admit) then some way must be found of raising the standard of the festival programmes as a whole. Concerts and recitals such as these, which never arouse more than a flicker of real enthusiasm, are bound in the long run to do a disservice to the cause of contemporary music.”
— Jeremy Noble, “ISCM Festival,” The Musical Times, Vol. 103, No. 1433 (July 1962), p. 465 & 466.
“Chaque année la Société internationale de musique contemporaine (S.I.M.C.) procède à une ‘exposition’ des œuvres sélectionnées par ses différents comités nationaux. Le choix est donc très divers, car il y a des pays à ‘musique forte’ et d’autres encore ‘sous-développés’, mais il est rare que quelques œuvres de valeur n’émergent du lot.”
(“Each year the International Society for Contemporary Music (ISCM) holds an ‘exhibition’ of works selected by its various national committees. The choice is therefore very diverse, as there are countries with ‘strong music’ and others still ‘underdeveloped’, but it is rare that a few works of value do not emerge from the lot.)
— Jacques Lonchampt, “Le Festival de la S.I.M.C. révèle ‘Antiphonie’ de Henze,”
Le Monde, 5 June 1962 (translated from French with the assistance of www.deepl.com).