Hmmm. I guess I don't see poly as being more risky than monogamy. (I assumed the 'crazy rider' person is the poly person and the risk-averse person is the mono person. This may not be a correct assumption on my part.)
Any way, both are subject to disappointment, relationship failures, people behaving stupidly or thoughtlessly. There is more opportunity for people to be people when there are more relationships, or potential relationships. I suppose that could be perceived as more risky. Still, I just don't perceive poly as being inherently more risky - the risks are pretty much the same as mono relationships.