I'm convinced the majority of "unicorns" ignore (or actively dodge) externalities in favor of lucrative profits.
I think most companies develop convenient ethical blind spots where profit motive conflicts with the ethical problem from generating externalities.
AirBnB takes a strong moral stand on "inclusion" as a principle but has a clear blind spot when it comes to consumer, property, and residential externalities. Facebook takes a strong moral stand on free speech, but it has a clear blind spot for privacy and public discourse.
Other companies don't necessarily take moral stands, but they still develop convenient blind spots. Youtube (Google) and ideological polarization in its recommendations, Equifax and its lack of cybersecurity measures, and so on.
I think Facebook expressly does not take a stand for free speech, they've been pretty clear about banning offensive/hateful/sexual content from their platform (to the point of being prudish, e.g. with female nipples).
It does?
Some investors would not be dissuaded by (2), if they are purely looking for financial returns. In fact, if you could make a compelling argument that you're externalizing some of your costs, it could be attractive to financial investors because it would help prove that your business model can be profitable.
At the end of the day, it may still be a good thing for the temporary housing marketplace if people who want certainty (in general or just for the trip) stay in a hotel, and people who are willing to trade certainty for some savings stay in Airbnbs. This would lower demand for hotels, and therefore cost. But this is only an efficient outcome if Airbnb guests understand how the company will leave them high and dry if something goes wrong.