What's the drawbacks of choosing a high statistical power and low significance level? I want to perform an AB test for two products (a control and a variant). I want to detect a 20% uplift with a very high certainty i.e., statistical power and a very low probability of a type I error (significance level). Let's say 99% and 2% respectively.
What is the drawback in doing so — choosing alpha and power to be very low and very high respectively, other than needing a very high sample size for both groups?

You identified the problem: large sample size required. That could get expensive, there could be ethical issues (e.g., human subjects), etc.
