Are they dangerous?
No, not really.
On rare occasions, swarms of these entities will attach themselves to one person, and will be able to physically affect them. As ever, this experience is generally harmless and pleasurable assuming the victim is healthy and suffers no strange repressions regarding sex. It can be rather exhausting to them, though. Under such circumstances, banishing and the use of appropriate prophylactic talismans is recommended should they subject wish to get rid of the beings.
Some mortal vampires and rather more vampiric magicians can project themselves into dreams in order to feed in a manner similar to that of succubi. It is also possible for corporeal vampires (of various sorts) and vampiric magicians to feed on sexual energy in just such a manner as the incorporeal succubi and incubi. Such magicians can also evoke, invoke, and summon succubi, among other entities, and can "create" one with a specific form to send to a chosen victim.
Incubi and succubi often form an entourage to more powerful entities, particularly those entities with vampiric and sexual associations.