Kolabnow is rather expensive and we had a lot of problems with it. The up-time was also very upsetting. Experience was 2 years ago.

Self-hosting of Kolab is not very flexible and a lot of work. We use self-hosted SoGO now and are very happy.

I recently joined Kolab to host mail and my calendar. Their customer service is so-so. They also have had some weird certificate issues that prevent my caldav from sync'ing properly - which is kinda inexcusable.

The one thing I like about them is privacy.

E.g. the employees don't have access to your mail. Period. Their policy requires them to have your specific approval to access your account/mail.