To preface this, I'm going to say that if the asker is going to use vague terminology like that and then leave out extra detail as well, then they have no right to complain if they don't get what they want in an answer. With that said though, I'd just take a very general approach to what these questions are saying. Interpretations like 'Is it better than a free slot?' are probably too methodical and I think deviate from the point. But they could be part of your response if you want to go to that extreme.
'Worth using' is probably the harder of the two wordings since it has a varying meaning as you've alluded to, but just assume the question is in objective terms and answer it that way. If I was answering that question about Unown, I'd say no, because its stats and movepool are terrible and there are far better Psychic-types available in the game. So I'd assess it based on what makes it appealing (or otherwise) and compare it with other similar options in the game. The most general interpretation of this would probably be, 'Is this Pokemon the best (or at least close to it) of all other similar options in the game?' If it weren't then it wouldn't be worth using because there'd be a clear better option available.
'Good enough' is a bit easier since there's little in the game that wouldn't be at least serviceable in-game if you knew what you were doing. Perhaps if there are better options for a team slot you could allude to that, but I guess the majority of the time the answer would be 'yes'. If you could make a case for 'no' like you might with Unown, you probably want to go into some more detail as to why.
I hope this makes sense. I understand the confusion, I think in a similar way lol.
