hi,
which version of Unity works with the least amount of issues with the current version of Playmaker
1.8.4 is fine so far and has a lot more actions and works good up to 5.6.2
I just tried installing 1.8.4 on a new 4.7.2 project and there i have no errors.
if you get errors, commonly it is because of duplicate actions.
some actions that where on the EcoSystem are now included in the standard playmaker (for example several Animator actions)
and that's why you should read the 'update check' carefully so you can remove them already before you install.
Should I play it safe and upgrade to Unity 4.7.2 or maybe Unity 5.6.2?
It depends a bit on what other assets and custom actions that you are using.
But i would not go to u2017.1 yet as there is a bug that prevent you from using the Ecosystem.
Update to 4.7.2 should not give any issues
But i would suggest to try 5.6.2
i am not sure if it is better to upgrade Playmaker 1st or to upgrade unity 1st
so it would be good to have 2 version of unity 1st and multiple backups of your projects to try.
but i am leaning toward upgrading playmaker 1st and then unity