Not totally safe, check Huk v Nestea from GSL Nov. It is safer if the opponent doesn't go gas first, if he does, speedlings are going to make your life hell.
If you scout a speedling opening get more zealots/sentries before expanding.. and if they make alot of lings - transitioning into a 3gate expo isn't very hard...
HuK played that game pretty badly - by his standards...
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