Adopting is just like giving birth: you don't know what you'll get
We actually feel more comfortable with our adopted child's health status because we met and interacted with him at 5.5 months old. We are now waiting to bring him home
Our 3 older kids all have issues we deal with daily: severe food allergies, asthma, developmental delay due to prematurity, speech delays, etc. We didn't know when we were pregnant that our kids would struggle with these issues.
Do we know that our adopted son won't have these issues? No, but we know he is developing appropriately for his age and that is all we have to go on. We do know more about him than we knew about our birth children though, if that makes sense.
If you are not willing to face the unknown, adoption is scary. I just don't see it as a bigger unknown than actually giving birth.