1. If you need variable times such as morning slots and then evening slots. The only way to do this is by creating two calendars. One would be the morning calendar and the other the evening calendar. Your customers would then have to go to 2 separate calendars.
2.If you need to customize the email per calendar. Only one email template for all of your calendars.
3. If you take vacations or need to block off certain days or times. There is not a way to block off times.
4. If you want to put all of your services on one calendar and have the time slots adjusted by time. To be fair, I am not sure there is any calendar on joomla that does that. If you have variable times for services, you would have to create a different calendar for each.
I love the interface and several other features, however, I am hoping that they will add more features in the next version to make the calendar more useful for me. I hope this was useful for someone :-)