If you learn non fluid design first it seems hard. In absolute terms, it's really just a matter of using flexbox and percents, and being willing to just scrap your design idea and do something else if it doesn't map nicely to something fluid.

Client education is a challenge and all, they often have ideas in mind that are very specific (For some reason clients invariably like the simplest thing that's the least general and most direct of a translation from a basic analog system...). But on the technical side... once you stop making designs that rely on being able to control exactly where everything goes it gets a lot easier.