When someone says liability for objects it means that object is not generating a profit or positive return but instead costing money or other things. When someone says “Are you a liability?”, they are asking if that person will cost them things like time or money or bring upon some type of loss now or in the future. Someone who is not beneficial to them but instead quite the opposite.

That depends a lot of context. Typically it refers to someone that is either a risk, or that they do actually cost you time or money, or are somehow detrimental. In a business sense, it could be someone who doesn't do a good job. It could mean someone who makes, or might make bad decisions that impact the company negatively, in either costs, or reputation. In a recreational situation, it could refer to someone who can't be relied upon to be an effective member of a team. They might make bad decisions, or fail to do what they need to do if they are under pressure, which might put the lives of other members of the team at risk.