Quote from Tripoint:
The fact was, in jump you were always naked to forces that you didn't
understand and that physicists couldn't measure because physicists
couldn't measure without instruments, and instruments didn't work
there, or at least didn't produce consistent results.
So there's no in-universe hard data. The descriptions/reports of how bad the consequences of jump without trank drugs are seem to vary a lot, from "anyone who does it goes insane" to "it's very unpleasant, but most people recover just fine". IIRC, there's several stories that show the latter (Definitely the first Chanur novel, but I think also Tripoint and Rimrunners), so the former can be seen as in-universe exaggeration. After all, spacer kids grow up being told how important it is to always take trank, so it's no surprise the dangers end up being exaggerated.
Tripoint has many descriptions of what jump (with trank) feels like, and it's basically very much like a dream or psychotropic drug trip, unpredictable and varying in intensity:
But kid or adult, the mind painted its own images on the chaos
It could certainly be described as "seeing things", but still based on what you did or thought about before jump - I don't think there's any indication for or even strong in-universe myths about the "things" actually being any kind of malicious spirits like the warp demons in WH40K.
The drug trip analogy seems to be pretty close, with trank having the effect of drastically reducing the intensity and danger of the trip. In reality, people do develop all kinds of psychoses after particularly bad trips. The main difference is that unlike psychotropic drugs, jumps don't become more dangerous when you do them regularly, at least if you take trank.