There are Fender and 3rd party JB necks with the top access truss rod so its not impossible - indeed the more moderately priced 3rd party JB necks I found last time I looked were pretty much all top access. I suspect it would be more likely to break if you dropped it rather than just in use. Having said that I wouldn't be comfortable with a top access JB neck, but I have zero evidence that its unsound.