I suspect drilling and tapping an aluminum stem is one of those things that usually comes out okay. I also suspect that higher-than-average failure rates were a lot easier to keep quiet back before the internet.

Personally, I'm not comfortable with "usually okay" on important bits like my stem. If I drilled and tapped one, I'd spend every ride thinking about it. What anyone else does to theirs, of course, is not my business - as long as they're not in front of me in the paceline.