Perennial Philosophy and New Age teachings reduce to healing. And while most people want healing (if they need it); healing is a negative goal, and thus is not a suitable basis for life motivation.
At best healing restores us to being comfortable and functional - and then what? That is the Big Question to which modernity (including its spiritualities of PP & New Age) has no satisfactory answer.
But a further problem (which good doctors know) is that there is a price to pay for healing. And the way that one-ness heals is, I believe, by a kind of ultimate nihilism about mortal life (because all is illusion and nothing really matters), which often leads to demotivation and even despair.
This is the danger of preaching one-ness. If you believe that one-ness/ PP/ NA works for you personally, then I think you should keep it to yourself - and not try to persuade others to adopt it.
After all, if you say you believe that other people's troubles are illusory, and don't really matter - because mortal life does not matter - then why do you preach at all?
It is this incoherence at the heart of any philosophy of one-ness which leads to the evil effects of one-ness teachings - evil, that is, from a Christian persepctive; where evil means taking the side that is opposed to God and to God's creation.