Iíve never heard of or thought of a correlation between a seat harness and breaking booms. Todayís booms are built much better but not too long ago the biggest correlation to breaking booms had to do with harness line width. When you placed your harness lines close together you point loaded the boom and it was much more likely to break.
As an instructor I often have students that ask about which harness to get or why I use one harness one day and another later on. I tell them that a waist harness is easier to learn on, easier to put on and easier to use so it will generally be the preferred harness. I also tell them that no matter how well made or how tight a waist harness is, it will still ride up a little in a gust where as a seat harness will not. I also point out that a waist harness wonít fit all people-people with too much midsection or no waist.
Now, here is how that little thing about riding up in a gust affects your sailing. If you are sailing in a situation where you need to spill power the waist harness is great because the riding up will force the sail to sheet out a little whether you want it to or not. If you are sailing in a situation where you absolutely canít afford to spill any power, use a seat. Otherwise, use a waist.