That's pretty risky. If you have a few million dollars that you want to invest into a dozen companies, you're diversified enough that if you make good decisions, it should pay off.

But at an angel level he probably only has enough to invest in 1-3, even fairly small rounds. So it's not too unlikely that all 3 of those companies fail and he loses it all.

High-risk bets, even those with high expected values, are bad if you can't afford to make the same bet enough times.