I love that I can sendan email to a small list of subscribers who are interested in a certain type of information. This prevents me from sending the email to people who are not interested thus avoiding spam complaints or unsubscribing.
I don't like that there is no drag and drop functionality, because it makes it somewhat difficult for a person without technical skills to adapt to the system.
I can get up top of my email marketing needs with Convertkit thanks to their very useful analytics. The statistics provided by them has helped me make excellent business decisions. I can see at a glance if we are experience a dip in our email list, and make changes to increase my list. I can determine where my new subscribers are coming from and they use targeted list-building.
Converkit allows me to see the conversion rate (or how many visitors became subscribers for each list-building optin offer I create.