I have to start by saying that the customer service is award-winning and amazing!
So far, TidyCal is easy to use and does almost everything I need.
However, there is one big problem.
I can't control how late a client can reschedule or cancel. I have a 48-hour cancellation policy for one-to-one work and a 24-hour cancellation policy for online events. We contract this agreement before we begin working. Sticking to agreements is a big part of the work. Being forced by TidyCal to allow cancellations or rescheduling whenever the client wants is hugely problematic, unfortunately.
Besides it not allowing me to run my business how I want, and to keep the agreements I've made, it means that I might turn up for an appointment that has just been cancelled and not be able to fill the space with someone who might've wanted it. In group work, it disrupts the group to have a last-minute cancellation. Sure, sometimes these things can't be helped, and I deal with those on an individual basis.
It's also a bit tricky that refunds aren't put through automatically, but I can work with that.
The rescheduling and cancellation thing is a big problem though.