To analyze the skull, we consider the cost and the benefits. The effect of the skull is that each time you spend a charge, you gain a resource and a card. But anyone can spend an action for a card or a resource, so the real benefit of the skull is that it gives you the other one as well. This means the skull gives you one of whatever you least want at the time – if you really want a card, you could have spent the action getting a card even without the skull, the benefit of the skull is that you also get a money. If you wanted a money, it gives you a card, and if you really wanted an action, you aren’t going to spend that action on the skull, so the skull is useless. So the upfront cost of the skull is a card and an action to play now, and the benefit is to gain one of whatever you least want, in the future, for each time you use it. So using the skull 3 times is kind of breaking even, you would want to use it 4 times to feel excited about taking the card. And since the skull is useless when drawn late, if you draw it early you want to use it at least 5 times to think it is a good card.
In my experience, it is very possible for a combat character (in a 4-player party) who plays the skull early to pick up 5 charges – indeed, to pick up all the charges they can handle. The real limitation is the actions you have to spend to use these charges. The analysis above is that if you would have to spend at least 5 actions gaining cards and/or resources, after you start gaining charges, to be happy with the skull. And that assumes you would have been happy spending the actions even without the skull (otherwise the skull really isn’t practical at all). But in my experience, most characters are kept very busy, and don’t spend that many actions over the course of a scenario drawing cards or gaining resources – and when they do, it is often at the beginning of the mission when the skull wouldn’t have charges. It is a special sort of character who has that much free time during the mission.
The Decorated Skull isn’t the sort of card that is really going to blow anyone away with its amazing power, you aren’t missing much if you never use it. But it can be fun and useful for a very specific type of character – a dedicated monster killer (so you put yourself in the right place to get lots of charges) who can’t investigate (so you have free time) and who is in a party with plenty of ways to deal with monsters (so you aren’t under constant pressure to save your friends, and thus have more free time), and who doesn’t need the accessory slot for something else.