Okay, first let me put down the caveat that religious/fetish self-castration is very much not my bag baby, because I find it pretty disturbing, so my readings on it are very light. But as (inexplicably!) none of the Roman or Christianity specialists around here wanted to take this one I did some research just for you. :)

I can tell you, first off, that allegations of self-castration among Christians was not terribly uncommon in this period, and it was something early church leaders were pretty anxious about. Origen's famous alleged self-castration occurred a couple of centuries before your mention of the Valerians, for example. Also the Council of Nicaea in 325 banned self-castrates from being ordained, it was that big of a deal. So it makes sense that a condemnation of a random religious sect at this period as self-castrating weirdos has been recorded. So why was this apparently such a trend that it needed specifically to be squashed? Basically, the early Christian church loved celibacy, it was a big deal, that's the first thing to consider. The second thing to consider is that there were lots of eunuchs around at the time, and they were seen as natural celibates, combined with traditions of holy eunuchs in other religions, such as the cult of Cybele, and other areas in the Middle East. So you've got 1. rather a lot of eunuchs running around and having a social role unlike now so the idea of becoming a eunuch is not completely alien, 2. the perception of self-castration as a holy and worthy thing to do (to some people) already, and 3. a pressure to be celibate/preserve male virginity, so self-castration was conceivably a very tempting option for a holy man. Consider the theological condemnations of self-castration are mostly concerned with it being a sort of "cheat" out of sin, and that by castrating yourself you're actually not really holier because you must earnestly face sexual temptation and defeat it to be truly holy. So, to put it in gamer terms, if you castrate yourself, you're playing Christianity with mods.

I am a bit dubious about the claim that they forcibly castrated guests, it feels like slander. Like "Oh don't go visit that sect, buncha weirdos, oh and they will castrate you!" you know? It looks like you are correct that they weren't really recorded anywhere but by Epiphanius, Augustine makes mention of them too but it's based on Epiphanius. But I don't find the existence of a small self-castrating Christian cult in the 4th century to be very out-there, I don't think we need to too heavily question that they existed. I'll let you decide if you should take a pass on their next church potluck or not.

I found mention of the Valerians in The Practice and Prohibition of Self-Castration in Early Christianity (if you have a JStor account or are at college, I thought this was a very good intro to eunuchs/castration in the early Christian church.) Also Eunuchs and Other Men: The Crisis and Transformation of Masculinity in the Later Roman West, Chapter 8, also made mention of them and provides a longer discussion of eunuchs in that society + early Christian church. This is a dissertation, if you have access to ProQuest Theses and Dissertations (if you're at college most universities sub to this, it's a major one) it's downloadable.