Not sure how many individuals actually use this with launchdaemons, but apple rewrote launchd process from the ground up with 10.10, and in the process I guess decided to leave out debugging.
I have found some "workarounds" but I cannot say that they work as I have not tested them myself. Here is the link http://apple.stackexchange.com/questions/169404/set-launchd-log-level. You can try using the "examine" command in the launchd, but as of currently there is no other way to debug launchd.