Is this "just" in the sense of "easily", in the sense of "only", or "barely"? The difference between "my amazing device just works!", "my employees just work, I wish they'd relax", and "that guess just worked".
Not 'easily', but 'only' – working is the only thing they do. It could imply some disappointment, but I don't think it's necessary. If you say about a device that 'it just works', you would change the verb in Swedish and say den fungerar/funkar bara.