In this area of Indiana, sloped brace mortises are the common rule (though you do find a few square mortises) When this is the case, in all such barns I have surveyed I do not often see the brace mortises as a location of post failure.

Usually, post failure in our barns occurs at the foundation -especially when the old boulder foundations were 'upgraded' to concrete footers, and the concrete causes the bottoms of the posts to fail. It is also fairly common to find posts failing where a through mortise holds 2 heavy girts. But post failure isn't the issue I see most of the time, usually I see roof failure and plate failure, resulting in a structure becoming unstable and the forces becoming unbalanced, causing the structure to tilt and collapse (and leading most people to incorrectly identify the problem as a foundation issue)