Alas, many people short-change not only their clients, but also themselves by taking on work that they aren't suited to. That's no way to build a long career, or to get recommended.
I see it happening with speakers, especially those early in their career, who understandably want to speak as much as possible. Of course, it's always possible to bluff your way through a gig, but how authentic is that? Another approach, which I see used across professional services, is to respond to a request by saying "I know you asked for that, but what you really need is this - and that's what I do". In my opinion, that's just as bad, if not worse, since it's suggesting that the potential client has no idea what they want.
So if you're trying to be a Jack or Jill of all trades, think again. The most successful professionals are those who specialise in a small niche, so that they not only have a huge body of knowledge, but everyone knows what they do. They are the people who get recommended. They are the people who can charge the highest fees, because they offer the highest value. So find your niche, stay in it and become the best there is. Then you will never again have to take work that you really shouldn't do.