The ability to manage multiple campaigns at once is great for people who are working with larger lists, also being able to reach out via other methods besides email can be very helpful when trying to get feedback from your audience.
I have found that if you send too many emails they will end up getting filtered as spam or ignored. If someone has not responded within 24 hours than it's likely something went wrong like their computer crashed etc., so make sure you give them time before sending another one but still keep an open channel of communication after this period of time passes without response. We use Contactually solely because we love how well suited its platform works compared to others around us while allowing users to create content easily through our website - which was difficult using some competitors platforms such as Mailchimp.