A lot of prompt engineering goes out of date quickly. Nobody nowadays goes "you are an expert software engineer. make no mistakes" lol.

As a personal anecdote, I find that a lot of big prompts and skills use up context window budget and in many cases agents will eagerly try to use a skill even if it isn't super relevant or necessary for the current task. So when I have too many skills I have to spend a bunch of time toggling the checkboxes to figure out which ones are needed for the task at hand before starting...