The requirements of the Unifi Controller do not allow a direct installation. But with a few manual steps a successful setup is possible!

Basic requirements

apt install wget gnupg2 ca-certificates apt-transport-https dirmngr gnupg software-properties-common multiarch-support

Enable installation of MongoDB 3.4

wget -qO - |  apt-key add -
echo "deb jessie/mongodb-org/3.4 main" | tee /etc/apt/sources.list.d/mongodb-org-3.4.list

Install old libssl required by MongoDB

dpkg -i libssl1.0.0_1.0.1t-1+deb8u12_amd64.deb

Install JDK 8

add-apt-repository --yes
wget -qO - |  apt-key add -
apt update && apt install adoptopenjdk-8-hotspot

Set JDK for Unifi

echo "JAVA_HOME=/usr/lib/jvm/adoptopenjdk-8-hotspot-amd64" > /etc/default/unifi

Install Unifi Controller

apt-key adv --keyserver --recv 06E85760C0A52C50
echo 'deb stable ubiquiti' | tee /etc/apt/sources.list.d/100-ubnt-unifi.list
apt update && apt install unifi