When adding a condition to a Flow step, you can only choose if A then this or if B then that. Just like the smart list, you should be able to choose if A and B then this or if B and C then that.
One example: If I want to both throttle and A/b test an email equally, I can't do it all in one step. You can only choose either if email address starts with A,B,C,D, E then send email X. But what if I want to add the condition if random sample = 50% send Email X. Theoretically, you can A/B test a throttled campaign but not random sample them equally.