Have you ever read the 3.5/PF module Carnival of Tears? It's effectively a primer on how the Unseelie can kill people for laughs. Notables include polymorphing, spiking drinks with potions to induce permanent Tree Shape, putting so much glamour on someone they don't notice being slowly killed by a torture device, being frozen in ice, having people equally glamoured so they don't realize they're killing the people mentioned above when doing various activities, giving people rewards that cause curses (stench, unluck, weird appearance quirks), and imprisoning loved one's souls in crystals for no real reason.
And that's when NOT in their realm. I bring it up because you asked about consequences for trusting them, but once you get into their realm GeoStationary gives a pretty good summary.
Other things I've come across in books and such include that they can give out a ridiculously harsh punishment merely for implying something insulting or a small infringement of manners. Names are important; don't give them your name. Plus, never accept a gift. If it's a trick, sad for you, but if it isn't, and they did not specifically say they want nothing in return, you just got a minor item in exchange for an unspecified time of servitude. As mentioned, never accept food or drink. (Cold) iron hurts them, but you may not be able to bring it. If there is a path, it is actually often trustworthy, and straying from it will invariably endanger your lives, but they will attempt to convince you otherwise.
The important thing is to emphasize the need for control and finesse and the sheer presence of the beings. Unseelie are powerful enemies, but can't be handled by waving a sword, even a CI one. Moreover, they are malicious, yes, but they aren't evil the way a devil or demon is. They'd kill you because they hate Goodness or want your soul. A Fey will kill you because it'll get them a cheap laugh.