a) Almost every startup I know of that failed, failed because of business reasons, not tech. Even when it was tech, the reasons were delays in feature delivery and production issues, not code maintainability or tech debt. Some companies paid dearly later on to fix tech debt, but if they hadn't moved fast in the first place, they wouldn't have had customers to lose.

b) This really depends on having a combo of a product manager who appreciates technology and an engineering manager or CTO who appreciates business. You have to weigh the benefit of shipping feature X now vs. later, in favor of tech debt T. Both sides need to be honest about the consequences of delaying X or T.

c) Not a YC startup, but always try to write tests and good code. Never abandon it. But in the early days when you're trying to gain traction, don't feel bad about having to compromise on them during crunch times (which is most of the time).