Well at a bare minimum, adverts should not contain content that would be moderated if it was included in a question or answer. This includes:
- Inappropriate or NSFW imagery of the kind that would be put in a spoiler block if it was needed in a question or answer.
- Scams or misleading claims that would be illegal in print advertising in countries where this is regulated (there have been a lot of examples reported of fake-medicine "snake oil")
Many SE sites are professional and used in a work context. A bare-minimum rule of thumb should be, allowed ads should never cause an office worker to think "I hope my boss doesn't see this trash on my screen".