Conventional sources of energy are usually non-renewable sources of energy. They can be harmful as most of the conventional energy are only fossil fuels. And fossil fuels are a form of Carbon, like petroleum and coal. By using these conventional sources of energy, we have two harms :-
1. On burning these rich carbon containing substances, we are adding gaseous Carbon Dioxide, Sulphur Dioxide, Carbon Monoxide and other toxic products. These are the causes of acid rains and global warming. So in one way, indiscriminate use of such technology is indirectly harmful.
2. These can be useful for the upcoming generation, and so, we need to conserve it, which is much difficult as the global needs are seen to date.
More precisely, all the conventional technologies are limited, and limited things never do the best.