What are the potential downsides to using more efficient and renewable technology?

For starters, most renewable energy sources are not "more efficient". A market economy tends to maximize efficiency (in terms of resources and labor needed to get it). The fact that renewable energy is not widely used reflects the fact that it is less efficient. So the major (some would say the only) disadvantage is that renewable sources are less efficient.

There are concerns with specific kinds of renewable sources, economic, and environmental.

Wind power turbines are hazardous to migrating birds, Hydroelectric impedes migrating riparian species.

Centralized solar power requires huge tracts of land, sometimes in environmentally sensitive areas.

Ethanol requires massive farming operations with all the attendant pesticides and land conversion, and has the additional negative consequence of driving up the price of food grains.

We've had hydroelectric dams for decades, but the eco-terrorists and fishermen downstream complained about some little fish or frog, so we had to tear down the dams & build unreliable windmills or nuke plants.

While environmental responsibility is necessary, we have to not consider humans as purely an invasive species to be shunned, but rather make measured trade offs.