Good question
In my opinion, the main difference is the non-cyclic character of the Windshear. Gusts, on the contrary, are cyclic:gust-lull-gust-lull. In a windshear, however, there is before the WS and after the WS.
WS has an effect in the IAS of the airplane that lasts enough to make things difficult, specially if the IAS decreases. A gust is always transient, it passes quickly, probably shortly followed by a lull and another gust. IAS fluctuates up and down. Thay make things more difficult, too, when they are intense, but not in the way that a lasting loss of 30 or 40 precious knots can.
I hope I've been helpful