Most important thing whether US investors participate or not is the way the token sale is structured. ideally it does not have a security character. If it is a security token, the SEC will sooner or later part of your projects. Howey test is a good test to see whether your coin falls into this category or not. But even in some utility token scenarios it is hard to assume that the SEC will be always outside. Regulations change daily and so does any ICO have exposure to that risk.