Science

@SJ_Zero As a poly person myself I thought I'd chime in:

One is a scaling problem. Let’s say you have a couple. Then add one person. Now instead of one relationship, you have three to worry about. Add yet another person, now you have 4 relationships. Add another person, now you have 9. Have each new so get a so, now you have 30.

This rapid geometric growth in relationships presumes that everyone is in a relationship with everyone else within the polycule, and this is not often the case. When you make a friend, it does not imply that you're now in a friendship with everyone else they are friends with. It's the same with polyamorous relationships, many polyamorous people don't care to have a relationship with, or even know their metamours.

The resource thing also hits in other ways. A lot of women want children eventually, and ideally children require resources – space, time, money. In a monogamous relationship, a woman can monopolize a man’s resources, whereas under polyamory she needs to share.

This can work both ways, with a kitchen table/communal living sort of poly situation it can also mean more potential time, resources, and attention per child, with more adults looking out for the children's interests. The, "it takes a village," approach. Potentially more caregivers also means more socialization and oversight. In monogamous nuclear families there's opportunity for abusive situations to arise because of isolation, as only a small number of people truly know what's going on within them. I have a hard time imagining a Mommy Dearest sort of situation in a large poly household when there's a lot of adults around.
Some problems do arise though, especially when it comes to the issues of inheritance and financial support within a largely monogamous legal framework. Moralistic judges may deny custody because a parent chose a non-traditional relationship structure. Then there's the issue of inheritance and property rights, which is baked into monogamy. In fact I believe this is one of the main reasons monogamy is the default today; genetic studies suggest that monogamy might have evolved more recently, less than 10,000 to 20,000 years ago, right around the time we stopped hunter-gathering and switched to agriculture, when social stability was achieved by knowing who gets the farm.

You can split people off and say “hey, I’m going to just have time with girl 1 today and girl 2 tomorrow”, and that can set up a sort of firewall, but there’s a bit of a resource problem there where there’s only 24 hours in a day and someone’s going to feel left out or someone who needs more support won’t get it.

Very true! Love may be infinite but time is not. Scheduling is important and so is making sure everyone feels loved and included, and there's sometimes negotiations to make sure everyone's needs are met.

On the topic of kids, there can be a real problem if a woman gets pregnant. Whose is it?

This is also an issue when dating in a monogamous framework. If it matters, there's paternity testing.

Honestly, you can make it work, but it’s hard mode. It’s much more difficult to make polyamory work than monogamy

I would say it's different, easier in some ways, harder in others. One way it's easier: there isn't so much pressure to be everything to your partner, to meet all their needs or risk being left for someone who does. Instead, you can have partners that fulfill different needs without abandoning the last one. It's like having multiple different friends you do different activities with.
I'd say what makes it more challenging are the additional complications of more personal dynamics to work through; polyamory requires a lot of communication. You can't rest on your laurels or ignore issues, you always have to be maintaining both your relationships and yourself.