As of 1.10, a few furnace event-related features seemed to have stopped working.

As an example : using a hopper on output COMPLETELY DISABLES the 'luck' factor of the ovens (which is done using

#getInventory().setResult()

).

Here is the file where we use this method : you can see we aren't checking if an hopper is below the furnace.
https://github.com/TheBusyBiscuit/Slimefun4/blob/master/src/me/mrCookieSlime/Slimefun/listeners/FurnaceListener.java

Here is the issue that has been created on Slimefun's issue tracker. You may find there some useful informations :
https://github.com/TheBusyBiscuit/Slimefun4/issues/86