There have been thousands of bikes with similar mid blade H2O bosses for rack mounts that are still running just fine. The dozen or so of forks I've done this with worked fine for low rider self supported touring over dirt roads for hundreds of miles, besides the day to day riding. If the boss is nicely brazed I doubt there would be an issue. However if one's concerns are so big I suggest bypassing the M5 mounting bolts that nearly all racks are made for and go to M6 fasteners Of course this might mean making one's own racks...

IME the drive for a unthreaded through tube for a boss was started by Blackburn and their CL-1 low rider front rack that removed the rack brace loop that previously was spanning the tow rack sides, over the tire. W/o that loop/brace a rack will be far more wiggly under loads and steering motions. The CL-1 rack has a second horizontal rail that contacts the blades inside, thus providing some triangulation to the rack's mounting. I've discussed the benefits/drawbacks to these through tube bosses before and I don't think there has been any final conclusion that I had read that really answers the question to which mounting is "better".

I have read of a very few blades that failed at the mid boss location. Usually a front end impact causing the blade to fold back at this point. I don't remember hearing of a crack at the point starting though.

My early mid blade bosses were just as the photo shows. A H2O boss in a drilled hole. Found quickly that the horizontal rails really want to be stood off the blade so pannier hooks have space to overlap the rails. Of course there's short stand off spacers provided in the new rack kit and they work well enough, assuming the bolts are kept tight what with the increased bending loads the bolts see after the spacers are installed. But this issue wasn't what I wanted so I explored various boss mounts as i replaced my, and the wife's, touring bikes. nest up was a seat stay rack boss (those hourglass shaped ones) brazed onto the blade. Much like Frakenstien electrodes. The height/length of the boss provided the stand off clearance. This design has worked quite well but the surface only contact did make me wonder and for my most recent touring build I made a M5 boss that has a spicket that fits into a hole in the blade and also has slightly larger a base. These changes create more contact area for the silver to flow into. Here's a couple of shots of this. Andy