Synonyms for Ruinous (same or very similar meaning)
WordNet sense 1 (extremely harmful; bringing physical or financial ruin):
catastrophicWordNet sense 2 (causing or capable of causing harm):
harmfulWordNet sense 3 (causing injury or blight; especially affecting with sudden violence or plague or ruin):
blastingWordNet sense 4 (causing destruction or much damage):
destructiveFrom the ODE community, based on WordNetadd/edit