That can be true in some instances, but often it just means that there are steps which require significant time or needs the involvement of more people and obtaining their cooperation. "It's difficult" often just means that significant resources and time are needed, and is especially relevant for unbillable work. This often results in things not proceeding (an eventual "no") but that is not a foregone conclusion. Alternative phrases such as "this plan is unrealistic", "this sounds expensive and nobody will pay for it", "this needs approval from a high level and will take time and effort to convince 3 levels of managers", or "we have no capacity" are often the actual position, but politeness and succinctness distill this to "it is difficult". This is almost never the end of a conversation, just the initial indication of if something can or can not happen.