Lync hasn't worked well with any version of Yosemite. My MS reps are telling me that essentially nobody cares. (not in such direct words but same end result)
I can load Lync, but can't see desktop shares, when someone calls me it keeps ringing long after I've answered, preferences crashes the app, can't close the app down without force killing it... the list goes on.
And Microsoft wants to call Lync an enterprise solution. Time to look at WebEx or something superior.