I am not running them on jails, just plain FreeBSD systems, but I hope my experience can help.
The first time I installed them I used the available ports. However, the port maintainer doesn't track alpha or beta versions, just official ones. So I ended up doing everything by myself. Anyway the ports help to get dependencies right.
Some issues: mFi is not compatible with modern Mongodb versions. It requires a version of Mongodb in the 2.4 branch. I suffered serious problems with that when I migrated from Mac OS X to FreeBSD, as Mondogb had gone to 2.5 recently. The symptom was, it was unable to adopt devices.
Finally, someone in the Ubiquiti forums casually mentioned trouble with a mongodb version, and I obtained the port files for the latest 2.4. It did the trick, it's been working like a charm for a long time now.
I haven't seen any differences between running Java 7 or Java 8. I see that the mFi port has been discontinued, but you can find it going backwards in the FreeBSD version control.
The latest port for UniFi is for version 5.0.7, from June. My installation is working with mongodb 2.6.7.
For updates, I do the following: (I have a dataset for the UniFi installation, and a separate dataset for UniFi/data)
Code:
# service unifi stop
(wait for Unifi and mongodb to really stop)
zfs snapshot pool/unifi@whatevernameIchoose
zfs snapshot pool/unifi-data@whatevernameIchoose
cd /usr/local/share/java
unzip /tmp/UniFi.unix.zip # The file I have downloaded from Unifi
# and now time to fix a couple of bits
cd /usr/local/share/java/UniFi/bin
rm mongod #it will be a symbolic link to /usr/bin/mongod, which is so linux-like, hence wrong
ln -s /usr/local/bin/mongod mongod
cd ../lib
#
# the snappy library they ship includes binary support for Linux and maybe Solaris (I don't remember)
# but not FreeBSD. Just install archivers/snappy from the ports collection and make a symbolic link
#
rm snappy-java-1.0.5.jar # the Snappy library they ship has system dependent code, replace it
ln -s /usr/local/share/java/classes/snappy-java.jar snappy-java-1.0.5.jar
After doing this I can start the service without problems.
# service unifi start
Been running like this for some time, and I even had to rollback once. Just a matter of stopping and rolling back snapshots.