I disagree with that statement. (Most) Hallucinogenic drugs aren't inherently unpleasurable; drugs in western culture have, over the course of recent history, been demonized and are now widely viewed as unpleasurable through socialization and conditioned behavior.
If hallucinogenes were truly and utterly unpleasurable to the human psychology, then people far and wide would not feature them as prominently in their various cultures as they do. If hallucinogens weren't pleasurable than you wouldn't expect there to be periods of time like we saw in the 1960's when, according to surveys, as much as 1/2 of the US's young adult population admitted to using them more than once.
For the most part, the only people I'd anticipate who would not find any hallucinogenic drugs enjoyable to even the slightest extent are those individuals inauspiciously predisposed to any one of the numerous psychiatric disorders which can be triggered by their usage.
That said, I do concede that there are certain hallucinogenics that are utterly unpleasurable. These drugs, however, do not have any type of widespread religious or leisure-centric usage. This group includes are ton of Rx and OTC drugs, which when used for medicinal purposes, are indicated for use at doses significantly lower than their psychoactive thresholds (eg gen 1 anti-histamines and OTC muscle relaxants). Despite being more or less physically safe when given at doses high enough to make 99% of people trip their nipples off, these drugs aren't hidden behind legal barriers and scheduling status because no sane person in their right mind takes them consistently as hallucinogens.
Hell, Salvia still isn't illegal in most states for this reason. That drug has literally no usage outside of tripping, yet it is so widely recognized as an unenjoyable trip that there isn't any type of rush to ban it. This wasn't the case for 'bath salts'. Most of those drugs were actually pretty safe derivatives of drugs that have been widely popular for decade, and the reason most these derivatives were given emergency scheduling status has less to do with their safety than it did with people worried about them being too enjoyable. This was the reason explicitly given by more than one state legislative bodies as justification to enact laws that permanently placed designer drugs on schedule I status in their state as many worried the DEA order wasn't enough to stop people from using them.