I drilled my GB "Forged Hiduminium" stem in the mid-'70s and have ridden that bike ever since, including light off-road, but low miles. It's mostly been a commuting and errand bike. I am a sprinty clydesdale+ and I used to sprint away from most stoplights, so I probably have stressed it more than almost anyone short of a Keirin racer. Nowadays, having broken one pedal axle and two crank arms, I accelerate more gently. Bing older, I'm more breakable and less immortal.
I have also drilled a few other stems, but no others with that many years of "testing". On my bastardized '73 Schwinn Super Sport, I have a cheap Hissing Lung stem that I drilled "front and center", right atop the handlebar clamp area. Once I knew what handlebar angle I wanted, I just drilled and tapped straight thru stem
and bar, with the added benefit that my bars are less likely to slip now! I don't necessarily recommend that, since I have seen a stem or two that broke there, but this particular stem has "plenty" of meat there. I am not worried at all. (Could be my epitaph?) But my opinion is based on engineering training, and decades of metalworking experience.
That's only been that way for maybe 10 years now, not as much testing as on the Hidumium stem. I like how the bell is right under my index finger without taking my hands off the bar, without using up any precious handlebar real estate.
Mark B