Can I ever be content? Yes. Is there a limit to how many times I might fall in love? I don't know.
I was with my husband for 5 years, identifying as polyamourous, before I fell in love again. Then only a few months later, I fell in love AGAIN.
Right now this third man is my boyfriend, and the second one is "just" my love interest. I have feelings for him. I hope things might develop further. I'd be glad to be just friends, too, as long as we stay in touch.
For me, there is no "I'm not content, I'm looking for more people to fill a void". I've never looked for people, whether I was single or not. For me, it's more, you go through life, taking care of your current relationship if any and savouring it, and then you might fall in love with someone else, or be attracted to someone, depending on the kind of relationship you're interested in I guess. I'm not one for casual sex, I need a deep emotional investment, I need "serious" relationships. Whether it end up being a live-in thing or a long-distance, see you a few times a year kind of thing, I want to enter a relationship with no doubt that if I can help it, it will last for a very long time if not forever.
I can't personally imagine dumping someone after the NRE is replaced with ORE. I have to say, though, right now I am experiencing limerence, NRE and ORE with the three men I love, and it's interesting to feel all three at once. I wouldn't say one is better than the other though, they're all different, and ultimately what matters to me is the people, being around them, spending my life with them.
So I guess I can see how NRE can be addictive and why so many people, poly or not, get "their fix" and move on. And I feel it's fine if it's okay with everyone. Less so if you take people and dump them when you're done with them.
To reply to the other question, the challenges... I would say that a mono/poly relationship is harder than a poly/poly relationship, too. My husband is mono and I often feel like I neglect him. He has mentioned being very happy for me but at the same time a bit envious of my NRE and hoping he could experience that too.
Actually he is starting to wonder if he might be able to be happy either way. It puts me in a weird place, since on the one hand I know it would be easier for our relationship if he was poly, because we'd be more on the same page, on the other hand I don't want to encourage him to become something he might not be. So he really needs to figure it out on his own.
I think the main challenge when you are a poly person is telling it to someone you love, whether you're current partner or someone you're interested in. Because there is always the risk that this person, that you care about so much, will think less of you, or be hurt, and that's a very scary thought.