macOS Server is very much a disappointment. The main reason for me to use it is because of the quick setup of Calendar, Contact, Notes, DNS and wiki which syncs with rest of the Apple devices when within the internal network, without having to install, configure and fiddle with these services separately. With each new update of the server app, Apple keeps removing features, which begs the question, is it going to be discontinued, which very much likely is.

Server 5.3.1
macOS Server 5.3.1
Server 5.5
macOS Server 5.5

Update: With the latest 5.6 update more services are being removed to prepare for the migration to alternate services. The support article HT208312 lists the following services to be removed in fall 2018.

DHCP, DNS, VPN, Firewall, Mail Server, Calendar, Wiki, Websites, Contacts, Net Boot/Net Install, Messages, Radius, Airport Management

Apple recommends to install those services separately rendering the mac Server app useless (to me).