Well, we may tell that modern smartphones are really vulnerable and there is nothing you can be 100% sure about them. First of all you should know that such a remote blocking can be used only by Google account owner, to which the gadget is attached. The problem is that gadget attaches to any Google account you have ever logged in with it. So, if your account login and password will be stolen – you won't be able to use that feature, but that feature may be used against you. That may happen if you use public wireless networks, Wi-Fi is vulnerable and hackers frequently use that.
So, what can a user do with that remote blocking feature, available from the Google Play Services? First of all you can set up a password and block the device remotely, any previously installed passwords will be erased and replaced with a new one. Also, you can track the location of the phone via GPS, or cell tower triangulation, if GPS is not available. But, all those features will work if the phone is connected to the web. The password will be replaced when the phone will connect the net.
So, if there is a universal cell phone jammer nearby – the feature won't work. Keep that in mind, because it works both ways, a thief may use it to literary “hide” your smartphone, or you, as an owner, may use it to prevent unauthorized blocking of your gadget. So, an answer to your question is probably “yes”, and it will remain so, until that feature may be used against legitimate owners.
Thanks for your question, I hope, I've managed to help!