i had my hood pierced, it was a vertical hood piercing just as you describe. the actual piercing was painful, sure, but not as bad as my nipples. sex for the next 2 weeks felt awful... any movement there was like a bee sting. it was hot going without underwear and it catching on my jean zipper as it healed... but once the 8 wk period was up, i was over it. the little silver ball was just getting more in the way of stimulation on my clit, rather than stimulating it.
i took it out after 8 wks.
anywho, i have to imagine that even if the ball were not smooth and rather textured, therefore constantly stimulating that nerve, it would desensitize it, at least while the hood was pierced.
there is also always the possiblity that something could go wrong. unless your piercer is super experienced, they could damage your nerve. then no more clitoral feeling. ever again.
and it would hurt like a motherfuck.