here's the problem the way i see it:
when you brake, you exercise force to the brake pedal and during the braking process the force to the pedal is there (the whole time) and transforms into pressure inside the braking circuit
with the line lock, basically you block the line so the pressure is held within the braking lines, hoses, calipers etc.
since liquids are incompressible any loss no matter how small will result in significant loss of pressure. Also, temperature changes that would result in dilatation of lines/calipers would also result in pressure loss and since there is no accumulator for the pressure or like during braking an external force to the pedal and in turn to the master cylinder to compensate for any of these the braking force at the wheel will drop and you're off to the races.
In case of a gas a minor loss won't be as significant like in an air compressor tank for example where if you have a minor leak the pressure will not drop abruptly but slowly in time, in the case of the brake fluid the drop in pressure would be instant and the only place that is still held/accumulated is in the mechanical deformation of the lines, calipers brake pads/shoes and hoses.
I also think (if i remember correctly) that there is a warning on all line locks to not be used as a parking brake
