Well, design is great but not 100% reliable, since here we have a relatevly thin layer of explosives that may not fully detonate or fail to do so completely. Secondly, it is detectable by metal detectors. Yes, I agree that aluminum foil will work as a cover, but still this bar may be suspicious to the security staff once they will try to examine it closer. Weight will be higher than for normal chocolate bar. So, this thing is great for low-moderate stealth terrorism like in schools, but not for something serious like an airport.
Note, that I do not encourage creating and using this bomb for terrorism, I am just giving my evaluation as a person that moderately familiar with explosives and security systems.