Okay, so what's the down side?
The down side is that you have more than two people involved in your relationship.
That is both a blessing and a source of stress. Romantic relationships come with a certain amount of tension built in; I've never known anybody , anywhere, who's never had even a single argument with their lover.
Add another person to the mix, and your potential for disagreements and arguments and tension gose up. A lot. Add to more people to the mix, it goes up even further. The more people you have involved in a romantic relationship, the greater the potential for problems.
It's not necessarily all bad. Sometimes, having people who you can turn to when you have problems is a big blessing. On the whole, however, managing more than one romantic relationship is, not to put too fine a point on it, harder than managing only one.
It is incumbent on any people in a poyamorous relationship to take care that they follow the rules, and make sure everyone's needs are met. Without that, the relationship will fail-just as a monogamous relationship will.
The other downside is that being poly emotionally riskier. The more people you open your heart to, the more likely it is thatyou'll have your heart broken.