Something that Austin and Jack talked about in the Introduction to Realis episode is that over the past few years they spent a whole lot of time talking about tokens and iterating how they work. As a GM, I absolutely love that players can discharge their token to use a Moon Sentence: it rewards them for playing into the theme of the setting and give us the opportunity to talk about them more. So with that in mind, I want moon sentences to be easy for them to use.

But I also know that moon sentences are an important way for the GM to throw wrenches in the PC's plans, and to give strong identities to enemy factions. A sentence like "Secret Police on [moon] always..." is extremely dramatic and descriptive of what a moon is like, but probably very hard for the PCs to use as their own means. The purpose of it is to give a specific view of who the bad guys are and what they do.

So: What are you thoughts on how easy/typical a moon sentence should be for the PCs to use? Is the answer just that it depends, and some stories want sentences to be easier or harder to justify using? Personally, I love when a sentence can clearly be used by or against the PCs: one of the sentences I'm most proud of writing is "Arguments on Areteas are always won by whoever speaks most," specifically because I can imagine it being used by everyone at the table over a game - or having a player intentionally never use it.

I think a Moon sentence should be just as easy for a PC to use as for the Opposition, since the GM gets to work with the invoked Sentence after it is used. Even if the players use a Sentence mechanically in one scene, the responsibility of the Moon's setting/storytelling moves back to the GM after the conflict, and they can use the previously invoked Sentence in that scene narratively, and later mechanically themselves, with it being a callback. 

A Moon sentence is meant to broaden the Moon's sense of place, like a unique twist that could only really happen there, and that means the Moon Sentences should be to either side's benefit. Your example of "Secret police" is a double-edged sword waiting over everyone, since it is the Moon's police, not necessarily the opposition faction's police. The secret police could just as easily arrest an opposing agent as a PC, and that's part of the drama. But once a PC invokes a Sentence in the story, that's become part of the story now, and after the mechanical dust has settled, the GM can apply that sentence in the narrative in other ways the PCs can't. The secret police could question the PC after they arrest the Opposition, or start tracking the Band more closely as an interested party, or remain involved as an interested party with their own goals separate from either of the first two groups. Once the scene has changed, and when narratively compelling, the GM can invoke those same secret police Sentence, and while mechanically it might not be a solid move, it'll narratively have more punch. 

I think as long as you keep in mind that it's the player using the Moon Sentences, not the character, you can make them as difficult to use as you want and it'll be fine. I'm thinking of the part where Jack uses a sentence that's something like "The casino always catches cheaters" (been a few weeks so I can't recall the exact wording) to have the bouncer catch an unrelated cheater, which lets them sneak in while the bouncer is occupied. So, "Secret Police on moon always arrest their target" or something like that could still be used by the player in some creative way. Maybe having the secret police arrest someone else, or maybe the PC wants to be arrested to further some other goal. A better sentence would probably have a more open-ended verb than "arrest" so the player can find a loophole.

Ideally I think the Moon Sentences should encourage the players to use them in clever ways, and one approach to that is writing the sentence so that it isn't obvious how the player should use it, which forces them to find a clever way if they want to use it at all. But I think "Arguments on Areteas are always won by whoever speaks most" is really good even though it's very clear how the player might use it, because it's also very open to being used in unexpected ways and it demands the player use it creatively even when it is used in the expected way.

A thing I thought was interesting re: this, as I was building out my ideas, was that I think Moon sentences should be the broadest and most open for PCs to insert themselves into - a situation that might work for or against them, but one they could easily find themself within.

Because there's a whole other set of sentences you can create for factions and NPCs, and _these_ are the ones that are more clearly oppositional. Now, how you differentiate these is really the trick; I don't know that I have a good way to do it, but one thing I was considering with the bits I wrote was, if I could relate it to the Moon's impulse, it could be a good Moon Sentence. If it was more specifically targeted to a faction's goal, or seemed too difficult to use, I'd move that to an NPC and write something broader for PCs to play with as well.