An obvious concern would be whether excessive prevention or removal of AB renders the brain vulnerable to microbial pathogens. We have not yet seen this with experimental-stage drugs like aducanumab. And anyway an argument can be made that the trade-off may be justifiable, in older patients already exhibiting initial cognitive decline.
But this shows how medical drugs are not magic beans, which the laymen doesn't understand. It's why you need an MD to prescribe them, and an FDA to regulate them. And why investors in AVXL need to stay patient, vigilant, and cautious.