So what's the issue with perennialism, neo- or otherwise? It comes down to spiritual ultimate and goals are not all valued equally in the major traditions. Some value awareness, some the Trinity, others more embodiment. Where I don't agree with Ferrer is his concern for sectarianism (or spiritual elitism) becomes pernicious. Yet, he clearly espouses a participatory framework that is very relational, and some metaphysical systems are clearly more relational than others. If we can't reconcile it all in a neat little bow, let's not diminish quality when we see it.
(In my view, the ultimate is relational. But that's another story for another time.)
Alasdair MacIntyre noted the need to situate oneself in tradition to know the standards of the tradition. In other words, you can't find a universal standard by which to measure traditions’ rightness. The standards are always internal to a tradition and therefore requires membership in that community, making a syncretic approach to all traditions untenable. As they say, if you believe in everything equally, you essentially believe in nothing.
MacIntyre argues that philosophy in general and ethics in particular cannot proceed by means of reasoning from neutral, self-evident facts accepted by all rational persons. And if our philosophical/ethical communities are incommensurable, then we need to accept that any attempt at perennialism will also fall short.
So maybe we can't create a synthesis of the Religion of Tomorrow. But taking Ferrer's and MacIntyre's perspective into account, it may be we are best served by situating ourselves at home in one path or possibly some hybridization approach. Truth and method can come from different traditions, but not all the traditions. (I've always contended to be open to Truth wherever it can be found.) Moreover, being in a tradition does not have to compromise our ability to hold a critical eye towards it. We can always have both a keen sense of irony and reverence towards any rock we stand on.
But I will leave with a quote from Nikos Kazantzakis that Ferrer mentions in his presentation that gets to heart of the matter of the perennial problem:
Within me even the most metaphysical problem takes on a warm physical body which smells of sea, soil, and human sweat. The Word, in order to touch me, must become warm flesh. Only then do I understand — when I can smell, see, touch.