Many/most critical issues are all backported to older JRE. Also Java8 is still receiving updates. There are MANY known Mac-Java issues that lead to crashes and the most annoying bugs are still not fixed after many years.
Our Mac installer already uses Java8,
http://jdownloader.org/jdownloader2
We also offer installer that uses installed java on system, you may try
mega.nz/#!nd800K7a!uvnqx5icsR_UHIheXhad3Q9BEYcsmGpFrHaIa77Abk8
Such installations will always use the system installed java version