I've used xbindkeys for a long time, but now on a fresh 16.04 Unity install it's not working. Running it in verbose mode produces this:
*** Warning ***
Please verify that there is not another program running
which captures one of the keys captured by xbindkeys.
It seems that there is a conflict, and xbindkeys can't
grab all the keys defined in its configuration file.
Any guesses about that "conflict"? (It's the tilting wheel button on a Logitech mouse.)

Does not happen on MATE 16.04 without Compiz enabled. It does happen on MATE with Compiz enabled, with xev *not* showing the button numbers it would otherwise.

So, I vote it's Compiz. But, is this new or is this just me finding out what's been broken all along?