Assassins actually start with very powerful ranged weapons - the daggers. Use these properly (as in, at all) and you increase the early game survival chances of your assassins tenfold. That should bridge the gap until you find proper missile weapons.
They are viable for everybody, no exceptions. Takes a bit of training, takes a bit of gathering ammunition, but the ability to hurt enemies before they can hurt you is priceless. Your melee weapon is for the cases when you need to stand your ground - and even this only if it's actually superior in damage.
ADOM Guides - whatever you wanted to know about playing a certain class, but have been afraid to ask!
Check out my youtube channel to see my ADOM videos, including a completed playthrough of the game. I try to give instructions, so if you want to see some place you haven't been before and get some hints on how to deal with it, this might help! There's also some other games featured there that you might find interesting.