But that is exactly the issue many of us have, which is that this is a resource which is of unknown value to a given PC. The player can only try to guess what sort of thing the GM will decide it is. They can't chose to select it vs something else, they can't decide to use it vs something else, they won't ride any stakes on it, because it could be worthless for all they know.
This kind of thing inhibits players from deciding to wager stakes and take risks. Instead they tend to just fall back on a small repertoire of well-understood tactics instead. Or more likely they 'play the DM' trying to measure just how much the DM is vested in a given outcome or scenario, or perhaps just using social engineering on them in some cases.
And what really is gained? The NUT of the motive was to let the GM, in a Gygaxian fashion, squelch 'abuse', which is really just a code word for "I don't like how easy this is to use." The 4e (at least) solution was to simply put things in a fairly narrow context, so 'Charm Person' generates a condition, and that has specific in-game effects. It can't really be abused because that's all it does! Now, that doesn't preclude other uses, but it puts them into things like 4e's 'page 42' (the rules for attempting things that are not already defined as powers or similar). Page 42 is pretty clear about what the expectations are there. In other situations you have SC rules, which are again pretty clear and give a good indication of the relative value of using a power as a resource cost in an SC. One of the issues with 5e is it lacks analogs to both of these, meaning you MUST leave things open-ended, and then you're always putting the GM in the spot of deciding if a given use is "OK" or not.