Only time I've broken one was a missed gear change. Stripped a gear, bound up then snapped the input shaft.
As for the weakest point I'm not sure, as I only have my personal experience to go by and I tend to have some mechanical sympathy unlike other people. Don't slam through the gears or dump clutches and rev match on down changes when at any decent revs.
If you want to be hard on your gear changing to an R154 (Think they are around NZ$3k new from Toyota) and a custom bellhousing (NZ$800-1000) would be a much cheaper option and proven to last, or run factory box, run to failure then worry about upgrading if it happens.