The default assumption should be that the rate of divorce will be higher. A couple has two relationships that can go wrong. A 3-person group have three relationships that can go South. A 4-person group have 6 relationships to worry about. Etc.
As far as, "beings could be more honest?" This sounds like poly-supremacist thinking. If you find yourself assuming any group of people is more innately honest or good, it's time to step back and check your privilege. This is just as bad as someone concluding that poly people are inherently less trustworthy or less capable of commitment, thus concluding that their divorce rate will be higher.
I think the divorce rate among poly people will be higher, but not because of any innate property of the poly people themselves. Each poly relationship is in fact many relationships. And every relationship carries the risk of breakup.