What are the drawbacks of protectionism?

stagnation in quality, efficiency, and innovation

prohibition of subsidies to domestic industries

preventing small businesses from closing down

increased prices and lack of product variety

risk of retaliatory protectionist measures

The major drawback in protectionism is the risk of retaliatory protectionist measures ruining the relationship of the two nations. This entails restrictions on goods of one of the countries. If those restrictions persist then the other ountry may impose barriers against the goods of the other country. This is exactly what is happening between US and China which is harming their economies.