For me, "complex" is a relative term. If I don't understand an involved process then I call it "complex" but once it becomes second nature it's no longer complex. All grain brewing started off complex but became easy. Winterizing my sprinkler system was once a complex process but is very easy now. I once considered brewing water design complex, but it's pretty easy for me now. To the level I need to understand these things to use them for the purpose I want, they're all pretty easy for me - BUT they didn't start that way for me or any person new to them.
Granted, with every one of those things, I don't need to know all of the science behind them to use in my daily life for the purposes I desire. I know about enzymatic actions at varying temperatures, but not all of them. I know about sprinkler line and valve design, but I don't know how electronic valves are constructed internally or the chemistry behind the polymers used. I know what levels of minerals I need to be concerned with in brewing and how to target them, but how the water constituents might interact inside a boiler at varying pressures and temperatures is foreign to me.