This question is like asking "what are the pros and cons of having three times as many permanent research positions that pay twice as much salary?" I'm sure you can come up with a long list of pros and only a very short list of cons, but the idea flounders on the assumptions. Where are you going to get the money from? I'll use open access journals to illustrate the problems since the journal's finances are clearer from the outside.
So you want to pay reviewers $100 per paper. Remember that you will need 2-3 reviewers per paper, and you will need to pay them even if the article is rejected. It's not uncommon for journals to have rejection rates of >70%. This means that for ten articles sent out for review, you need $2000-$3000 to pay reviewers, and you publish ~3 articles. If each article has an APC of $1500, then your net revenue is $4500, and two-thirds of your revenue (not even income) goes to reviewers, a level that is definitely unsustainable. Even for more expensive journals where APCs are $3000 per article, it's still ~30% of your revenue.
Given the publisher cannot currently pay for this spending, who is going to pay for it? One could:
- Increase APCs. Raising the APCs from $3000/article to $4000/article would keep the net revenue sans reviewer cost the same. But this would be a corresponding drag on the authors, taxing either their research grants or their libraries. I can pretty much guarantee that libraries are not going to like this. I can also pretty much guarantee that authors from developing countries will look at you with bewilderment and ask you where they're supposed to get the money from.
- Charge authors a submission fee. Basically the authors pay even if their article is rejected. This would also not be pretty. Absent the absolute best-of-the-best journals, you will instantly lose most of your submissions. And the money still comes from research grants or libraries.
- Charge readers to read the paper. That would make your journal into a subscription journal that charges an APC. It would also mean no more open access. You will again instantly lose submissions, and the money still comes from libraries.
- Start up a GoFundMe initiative to provide external funding to the journal. External funding is like manna, if you have it in sufficient quantities then everything becomes possible. If you can demonstrate that you are able to raise the money consistently, over a long period of time, I'm sure there'll be publishers willing to start a journal that pays reviewers (and stop once your funding runs out).
- Petition your local government (/Santa Claus) to spend more money on research, which would increase research grants and library budgets, as well as create more permanent positions. Yeah, one could do that. Every researcher will be happy if this is successful.
And then there is, if you pay your reviewers why shouldn't you pay your editors? It's hard to argue editors do less work than reviewers, they handle a lot more papers after all and they see papers that are never sent for review. If you pay the handling editor $100 per paper, and then the editor-in-chief another $100 per paper, then that's an extra $200 for every article that enters the system regardless of whether it's published. For every ten articles sent for review, you now need $4000-$5000 to cover your costs before you even get to production. From $4000/article APCs you now need $5000/article. Ask your library how their budget would cope if APCs went up by 60-300%.
Ultimately the problem is that scientific papers have extremely narrow readership. If you could guarantee that there will be tens of thousands of readers willing to pay to read your article, then I can also guarantee that publishers will be able, even happy, to pay the reviewers for your manuscript. Still, can you make that guarantee?