there are some differences in wording that separate whether you can play them in any combat or just ones you are involved in. "Play while in combat" often carries the limitation that it must be a combat in which you participate.
I think most cases of "play in combat" refer to any combat, though.
It's easier to work on a given example. Could you provide an example of a card that you argued about?
We found that a lot/all of the one use potions/weapons that add a bonus would say "play in any combat", implying that I can just throw it into a fight to screw someone over, by adding +3 to the monster.
Whereas the cards like "Ancient Monster" and "Intelligent Monster" that add +x to the monster, AND mean that if you defeat it you gain 2 extra treasures would read "play in combat". THe bonus of two extra cards does sort of imply that you play it only on your own fight to boost your treasure. BUT I have always played it that you can just throw it into a random fight, with the downside that if they win they gain more treasure.
Someone basically argued that they wouldn't have different wording if they meant the same thing.