What are the drawbacks of a liquid hydrogen jet engine?
Here are a few drawbacks:


Lesser energy density per volume


For the same amount of energy, you need more space, not yet available in current designs


Boil off


The hydrogen will slowly boil off, and vented to the atmosphere. Hydrogen IS dangerous, and this will negatively impact long flights.


Logistics


Fuel is common everywhere in the world and easy to store. We have yet to build a world delivery network for hydrogen.