The Israeli intelligence agencies, including the Mossad and IDF Intelligence Department failed to predict the rapid advances of Syrian rebels and the subsequent collapse of the Assad regime.
Yediot Aharonot reports that Israel had constructed a security mindset based on Assad being stable, with strong Russian support, and the Syrian army rebuilding - an assuption that persisted in the IDF until recently. This perception led Israel to court Russia and explore opportunities with Assad's circle to draw them away from supporting Hezbollah. The result has been several strategies based around a regime that no longer exists.
The rebels are a cause for concern in Israel due to Turkish President Erdogan's significant control over the movement. The rebel leader, Abu Mohammad al-Julani, intended to join al-Qaeda in Iraq following the second intifada in Israel. The Syrian rebels also believe Assad was supported by Israel.
Additionally, the rebels are less stable than the regime and will be more prone to abrupt violence, changes of policy and leadership, and changes based on outside influence.