This converter does not talk of "unregulated", does this mean it is regulated or did they just forget to tell me that?: https://www.xppower.com/portals/0/pdfs/SF_ITB.pdf
If you look at numbers beyond the headlines, both line and load regulation is specified on page 2. Yes, it sucks.
AFAIK, at least
some of these regulators include input voltage feedforward, i.e., some
line regulation, because it's easy to implement in their topology; but poor
load regulation, because it would require isolating the feedback signal from the output, which is expensive.
The simplest have poor line regulation, though, as well.
Actual "proper" isolated DC/DC modules that are properly regulated and protected can be horribly expensive and hard to find.
These cheaper modules are often indeed used to supply power to isolated communication transceivers, where the exact voltage, regulation and ripple tend not to matter too much. The efficiency tends to suck, as well.
Also note they are built to a price and certain footprint, and often require extra capacitance and possibly an EMI filter.