Perhaps a better question is "Do quad bikes present an unacceptable risk in a certain scenario?" All items from chainsaws to paper, carry some level of risk (those paper cuts hurt!) 'Dangerous' would imply a level of inevitability of an incident, versus risky meaning a skilled, trained and conscientious operator could use it for decades without incident.

Taking 'dangerous' in a simple context of the question, I would tend to agree that the level of danger depends on the use and comparison with similar farm equipment.
They're a lot less dangerous than the original Trikes, which had a habit of bouncing and rolling when cornering, but offer less protection than side-by-side farm vehicles in an accident. Much like other vehicles, they're designed to be used within certain operating conditions (speed, terrain, loading etc), for which they ought to be perfectly 'safe' to use. Use them outside of those parameters and the risk increases.
In agreeing with the statement, I'd repurpose a quote from 'The Castle' - but it's what you do with it!