I've made six or eight bows in the past year with "Z" spliced handles. I agree with PatM, the key is a well cut joint and the proper glue. I lay out my handles with the Z splice between 4" and 4 1/2" long depending on the length of handle I need. The splice comes out fractionally shorter since you can't cut to the very end of the "V" due to the band saw blade kerf width. I make the open end of the slice 5/8" and this layout has worked very well.
To help ensure I get a good joint, I cut the belly side of the handle area perfectly parallel with the back and square up one side and the end. This helps keep the billet flat on the band saw table with NO wobble during the cut. I also lay out the joint with a knife point, not a pencil, for better precision. A sharp blade and a steady hand and you can cut a good joint. A little clean up with a rasp and bench chisel and you're ready for the glue.
I found West System's G/flex epoxy from a boat building supplier and it has performed extremely well for these splices. It has a small percentage of filler in the glue and does an excellent job where you don't have a perfect joint.......which will be the case with a Z splice at best.
With a 4" splice you will have 12" of glue line.......a lot of strength. You will loose very little of the joint when you finish shaping the handle. Here's a pic of one I did last week.