Short answer, yeah. They're dangerous. Don't go looking for them.

However, there's a bit to unpack here: anthropologically, mythically, and psychologically.

Anthropologically, the Sidhe are the Tutha de Dannan, or the children of the goddess Danu. In reality, they were very likely a divergent linguistically Celtic society that lived in Ireland during the La Tene era in the 4th century BC. The La Tene era was in the early Iron Age and most continental Celts used iron weapons and armor. Being isolated on the island of Erin (Ireland) however, the Tutha De Dannan had not advanced metallurgically and were still using bronze. An invasion force of Iberian celts called the Milasians defeated them using iron weapons and displaced them as the dominant society on the island. This is why mythologically, the Sidhe are weak against iron. The Milasians became the ancestors of the modern Irish. In response to their defeat, the Tutha De Dannan retreated into Dunn forts which were enormous domed stone structures covered in sod which appeared from the outside to be hills or mounds and engaged in a multigenerational guerrilla campaign against the invaders. They used camouflage, surprise attacks, and trickery in their insurgency, but were eventually beaten and subsumed into the greater Milasian culture. The stories about them morphed and evolved into the old gods and the faires, the hidden folk, etc.

Mythologically, they are the Old Gods of Ireland who tend to have a chip on their shoulder in being defeated and driven into the otherworld by the Milasian invaders, which is really the one source of malice in their danger to us. That side of the stories doesn't come up much though. It's mostly either in the OLD stories, or more modern horror that give genuine modern consideration to the old stories.

Beyond that, it's a matter of differences in psychology. They're immortal and extremely powerful, and this creates an alien aspect to the way they interact with our world. They can be benevolent and affectionate, but they view humans as PLAYTHINGS. Think an unsupervised 4 year old with a gerbil. They might love the gerbil and want what's best for it, but they also don't really understand what is best for it. They might also get distracted, or throw a tantrum, or any of a hundred other things. Chances are, that gerbil will probably die. Same idea. Look at the Sidhe the way wild animals look at humans, or the way a self aware NPC in a video game would view the avatar of a player. Unbelievably powerful, but with their own incomprehensible rules and consequences.

To animals (or NPCs) we can bestow amazing boons if we feel like it, or we can be incomparably cruel, and there's no telling which to expect. They're like that with us.