Summer or wet season (November-March) brings heat, humidity and mosquitoes, especially in the Amazon where roads can be impassable. However, rain is less frequent in the Altiplano and everything looks lush and greener.
When traveling to Uyuni during rainy season expect to have a unique experience of seen the salt flats reflecting like the world’s biggest mirror. Downside is that a tour to visit the Fish Island will not be possible because of water on the salar.