However, beyond the flexibility of the scheduling, and beyond the comfort level you have with a prospective counselor, one thing stands above everything else.
Has your prospective counselor helped people gain admission to your top choice program(s) before?
If they have, great. Ask questions. What years, and how many people? Ask them if there may be anything different about your situation (note: there will be).
If they haven't, then naturally you are accepting some degree of risk if you elect to work with them. This, to me, is simple common sense. What have they done? If you are aiming for a top MBA and they have a lot of experience with top programs, then this could be alright. But what if they have little or no experience helping people get into MBA programs at all? Maybe in such a case their selling points include transferable skills that they feel will help you with your application. This may work or it may not, and the risk you assume is based on the idea that they will be able to figure things out and adapt well to this new situation as you work on your applications.
If you can't ask them directly before working with them, because a) you don't know who they are, or b) they or someone else won't allow it, then why are you even considering them as an option in the first place?