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The New Frontier Pass is a strange bit of content in that, ultimately, it's an expansion on the level of Rise and Fall or Gathering Storm that fans liked, (it adds new Civs and Leaders AND new optional modular gameplay mechanics, modes, and wonders) but it was distributed the earlier Australia and Poland individual DLC packs that fans hated.
Its final price is $40, which is definitely a bit overpriced when compared to Rise & Fall or Gathering Storm, which actually makes the piecemeal delivery worse, since you can end up paying more than that if you buy the content individually. You should definitely get this either on sale or as part of a bundle.
But is the content good? Well, yes. The thing is, it's also indecisive. The added civs are all generally overpowered in one way or another, but Firaxis seemingly gave up on balance with Gathering Storm so that's par for the course. But they are often fun to play with unique hooks.
The more important additions are the modular alternate rules that can be added to games. From adding super disasters to zombies to corporations to mythic heroes or more complex barbarians, they all change up gameplay a LOT. It's a smorgasboard of ideas trying to appeal to different audiences in different ways, and they end up generally succeeding. Personally I think the complex Barbarians that can become city-states and Corporations rules (both seem derived from similar systems in Civ 4) are absolutely essential additions to core Civ 6 gameplay, with the rest being fun variants. With the exception of the zombies mode, which, due to zombies being categorized as a barbarian faction, is just a reskin of the "Raging Barbarians" option with extra steps.
The thing is, that all these extra rules ARE modular, and ARE disconnected unlike the additional mechanics and rules added in Rise and Fall and Gathering Storm indicates that Firaxis just didn't feel very strongly that they were fundamental or necessary additions. And they all end up showing off that they're a bit undercooked in one way or another. Unlike, say, the complexity of Eras or Loyalty or the World Congress, these are just surface level takes on concepts. They end up adding flavor and novelty, but they don't add much depth unless you run at least 3 modular rulesets at once.
So in sum, the Civ 6 New Frontier Pass content is good, if 75% non-essential, and you should definitely only buy it as part of a bundle or for at least half of the base price. That said, the more Civ 6 you end up playing, the more these variants end up being a welcome change of pace, so I guess ultimately the pack did succeed for the core audience of Civ Fanatics when it initially came out.