Perhaps it has nothing to do with the actual person, and more to do with a difference in style of polyamory, or in communication styles? If relationship expectations are different, why wouldn't there be consistent issues cropping up? Let's say there's an introvert dating an extrovert, well, there are some different types of language that might be needed to bridge that gap, perhaps even with a "translator" person at some points.
Prevailing opinion would lead one to believe that the secondary relationship isn't the problem, it's just the spotlight that brings those issues to the surface. I suppose from the context of "picker problems" this could be true. Even if the metamour in question is a bringer of drama, someone picked them as partner material. Shoot, show me the poly person that's never made a poor pick.
When you've spent time and energy on a direct or indirect connection with someone, there is an investment in them. The desire to "make it work" is huge! So undoing "picker problems" is tough for all concerned. When I see people drifting from things they considered core to accommodate that, that's where a small difference can become a bigger issue.
Having strong personal boundaries is a good place to start. With poly, the difficulty can come where you implicitly trust one partner, but perhaps not the metamour on that same level. How do you maintain a boundary with the metamour without cutting yourself off from the fulcrum partner? I'm working on this diligently, so please bring on the insight!