It is certainly an oddball form of mysticism or mystical unity to imagine that truth or satya consists in recognizing that all things are temporal and finite. While this might be a mundane truth—mundane truths are certainly not union with the transmundane which is absolute.
On the same score, if Buddhists believe that it is somehow mystical to recognize impermanence (anitya), suffering (duhkha) and lack of self (anâtman) is all there is to this life, then they’ve failed at Buddhism. By no stretch of the imagination is such authentic mysticism. Why? Because it fails to acknowledge the transcendent which is not anitya, duhkha, anâtman. Let’s be clear about this, anitya, duhkha, anâtman only comprise mundane truth. They are not ultimate truths which only the mystical experience and tradition can reach.
Indeed, no amount of believing that all there is to life is impermanence, suffering, and lack of self is going to accomplish the transcendent nirvana. The Buddha only teaches this inferior crap because this is what the profane (prithagjanas) are really clinging to, which they are supposed to reject insofar as nirvana or the absolute is not impermanent, suffering, and devoid of self. The price of the desire that craves finite things that are always anitya, duhkha, anâtman, is endless rebirth.
Those who elect to take the mystical path of Buddhism, who are the arya-pudgala (spiritual persons) in contrast with prithagjana (the profane), are the only ones capable of winning complete nirvana and Buddhahood. They are well aware of the peril of glomming onto what is temporal and finite.