Currently, HTTPS support is not fully available. Links in emails will be HTTP, images in emails are served over HTTP, and when using a CNAME for web forms, the web forms will be served over HTTP as well.
It's possible to serve web forms over HTTPS by manually using analytics.clickdimensions.com, but that looks weird for our customers and still images will be served over HTTP, causing a warning in for example Firefox.
What needs to be fixed:
I'm especially concerned about the web forms. Our data protection officer has indicated that the forms should be served over HTTPS (because of personal data), especially with the GDPR deadline coming up.
Other ESPs do provide these services.