Rousette, I think I have as much - or as little 'emotional security' as anyone in any relationship. You can't guarantee the feelings of any other person 100% forever. People change, needs change, desires change, circumstances change.
I may choose to have a second/third/fourth partner at some point. So might OSO. So might DP. For example. But we have a relationship which can adapt to fit changing needs - it's one of the best things about it.
I do think poly relationships are harder work in some respects though, I'll give you that. You need to take into consideration more than one other persons feelings and needs and that's not always easy. But it means you have to be intentional about it, and do that very consciously. And I think that brings a lot of benefits and security actually. You learn to communicate, well and often and openly, because you have to. And you take few things as a given, because we 're not following a well worn path, we have to think about what we 're doing, why and how at every stage - it's never a case of "because that's what you do". If I ever found myself in a mono relationship again (and I don't think I will) then I'll take those lessons with me, for sure.
As for financial security, I'm perfectly secure all by myself, I don't need anyone's contribution. Our finances are linked because it's the fair thing to do when we share living space, food bills etc. And we're accounted for in each other's wills, for example. But I'm not dependent and intend never to be, on anyone. That's about freedom, and freedom in relationships is so, so important, I think.