Now sure, you're going to say, "I run one just fine... Lots of people have used them without problems... etc." Fact is, there have been many instances where they've failed or the wheel studs have failed. I'd rather be safer than sorry.
All of the failures I've seen were either cast spacers or improperly torqued spacers. Billet spacers with good quality studs are no worse than using aluminum wheels to begin with. Yes, you have two sets of lugs nuts to tighten per wheel. And yes, you need to check the lug nuts on the spacer/adapter. But only as frequently as you need to check the lugs on your aluminum rims.
As far as additional forces, the tire is going to end up in the same place no matter what unless you change the width of your axle. Even if you get a rim with a custom offset made, the tire's relationship to the hub is the same. That force is just entirely on the rim, instead of on the rim and a solid chunk of billet. But, there's more metal in a 1" billet spacer than is in the center of the rim you're trying to use, so, does it really matter? Not to mention, the aluminum rims are more than likely cast aluminum. Not billet. So, if you're worried about something breaking, worry about the rim. The billet spacer with properly torqued lugs is far stronger than the rest of the wheel.