* You have to pay yourself market rate, and thusly have to demonstrate capital in your business bank account. This is near impossible to do with YC and StartFund capital. Think: $100k per H1B employee. (and you need to demonstrate other parts of your business, such as an office lease - but these are hackable compared to raw bank statements.)
* You cannot be "the boss" what this means, specifically, is that you must be able to be fired. Have 3 co-founders? probably okay. Have 3 board members? probably okay. Having 2 co-founders is awkward. Good immigration attorneys can guide you through this, it's a reasonably hack-able problem to solve if you're smart.
Most people ignore the visa problem during Y Combinator. After demo day, they throw capital at the problem and fix it. Hire a good lawyer, push for an O-1 or H1B. Explicitly: once you've raised seed funding getting immigration sorted is fairly straightforward -- getting immigration sorted before raising seed funding is very very challenging.
What I mean by "ignore the visa problem" is they simply enter the USA under the visa waiver program once or twice. 99.99% of people have no problems being (but not working nor living, wink wink) in the USA for 6 to 9 months under this.
(speak to a lawyer. in fact, speak to a few. and have money. lots of money. and patience.)
For transfer, You have to raise at-least 75k+ per founder so your start-up can demonstrate funds for your salary.