First off I need to establish that I have several thousand hours of water flying under my belt in aircraft that put the hull into the water (not a float plane).
It us all about support points and the length of the hull. Think about a 10ft piece of timber moving over a 12inch pipe. The forward end will easily rotate downwards into the ground. In an aircraft on a single wave as pitch control reduces the same can happen with the nose getting buried in the wave in front causing extreme stresses and possible destruction. Now think about the same piece of timber over 3 or more 12 inch pipes. Much more stability. Experienced sea plane pilots assess the size of the waves, the period between them and the size if their aircraft {length of the hull) along with the strength of the wind and make an intelligent decision whether to land into wind and across the waves or parallel to the swell.
Unfortunately most pilots ditching don't have the relevant experience so the guidance in the pamphlets and documentation provided by manufacturers and CAA authorities is written in generic form. In my opinion it should be followed. The two points I would add to most of them though are
Speed really counts... Water is hard when you hit it fast.. minimum speed should be used but not do not stall onto the water.
If you know you will have to ditch and you can do it with power (fuel running out or your aircraft is burning) ditch with power. You have a better chance of picking your touch down point, minimizing the speed and controlling the touch down