Yep - what I have learnt is just cos someone claims to be poly, doesn't necesarily mean anything about them (that they are honest or open), other than they are in relationships with more than one person.
Oh I'd go further than that - I know people who self-identify as poly without having ever been in a relationship with more than one person - they have decided they are interested in this lovestyle but have no experience of really doing it - sometimes their reality is very far from their dreams (and this goes for individuals and couples).
Someone saying they are poly should be the starting point of a conversation, not the end-point. Don't assume ANYTHING about who they are, whether they are more trustworthy, less manipulative, less abusive, whatever. Anyone can say they are poly - literally anyone. There is no "polyamory certificate" that you have to pass some test for, or an organization with entry requirements.
There are people who use poly as an excuse to cheat on their spouses, to "get in your pants", or to "cull someone from the herd" (aka a cowboy/girl).
And there are control freaks and abusers.
...and there are also some wonderfully nice, stable people with their heads firmly screwed on.
So finding out exactly what they mean by polyamory is the next step - finding out what sort of person they are is the next - and both are vital before entering into anything with them.
And I've also learnt, that just because I may choose to be in a Poly relationship doesn't mean I have to accept being treated poorly.
Absolutely! If there was one bit of reading I could recommend (and it's not long), I would recommend the article
Polyamory for Secondaries from Franklin aka tacit. I feel strongly that everyone deserves respect and trust in their relationships.