Front brakes, either disc or drum ... are very vulnerable and should only be used in cases of extreme emergency. There are many reasons but I can think of the following
1) Normally the bike tyres are not "wet" tires (wet tyres are a type of tyre with more grip in water/rain etc) so if you apply front brake (whatever type) you have more chances of getting your bike slip
2) You bike is not in the upright position and you are slighty tilted to left or right and you apply front brake (whatever type), you will definitely slip
Hence people and mechanics tend to advise drum brakes because
a) They are NOT spontaneous (will not jam the tyre instantly like Disc brake)
b) Since you are already using DRUM brakes on rear tyre, you are more confortable with that and chances are that in case of emergency where you need to apply front brake, either you will push it very hard or very soft and it will not serve the purpose
Thats all I can think of right now