Christians can learn a lot from other religions; but those of no particular religion cannot learn anything substantive about Christianity - or indeed anything else.
Instead, these 'spiritual seekers' or adherents of the 'perennial philosophy' merely become trapped in a self-congratulatory/ self-indulgent version of the mainstream modern lifestyle - with its trapping of the sexual revolution, political Leftism, and technological self-manipulation of emotions (via drink, drugs, 'body-art', social activism etc.).
The perennial philosophy is therefore an amusement, not a conviction; an open-ended life-option that seeks no end and attains no progression; an interest, not a faith; and complacency, not courage. It is simply an addictive kind of feebleness - a craving for a pain-free and engaging mortal life - with genuine escape/ enlightenment always just around the next corner...
True seeking is based on the conviction that there is a right answer that leads to more right answers - it is serial finding, not serial seeking.
All answers - all true knowledge - are partial and biased (i.e. perspectival). Progress comes from the process of of integrating and correcting these answers - and it never ends; because reality is creative, so there is always more to know.
Spiritual seekers expend their mortal lives in looking-for a form - asserting that all forms and motivations are one. This is lethal to real spirituality; since it discards the ens and essences of metaphysics and motivation - and instead focuses on the means and peripheries of emotions, lifestyle, and detached/ fragmentary utterances and writings.
(A monk who seeks detachment-from the world is equated with a shaman who seeks absorption-into the abstract divine and a priest who seeks a personal communion-with a personal god... on the basis that they all wear robes and spend time sitting with their eyes closed...)
That the perennial philosophy is nonsense is a matter of simple common sense and logic; which is why the idea is only ever held by intellectuals who have the cognitive capacity to confuse themselves with complexity.
The perennial philosophy is at least tolerated, and probably encouraged, by the demonic beings who pursue strategic evil in this world, because it permanently neutralises the basic human quest for god, for true religion. Once enmeshed in endless syncretism and research - the spiritual seeker is trapped in a kaleidoscope of images and assertions, and cannot find a way out.
Of course, even sincere and genuinely motivated individuals will likely go through some kind of phase of comparison and learning about religions - but to defend this as a valid permanent option, to crystallise it as 'the truth' about the human condition, is fatal to faith, hope and charity - leading instead to this-worldly materialism, purposive hedonism and despair.