Most dirtroads are difficult after rain and often potholed. Generally speaking, the southwest (Uyuni) is pretty bad and very difficult with a roadbike (washboard, sand...). The lowland roads (Yungas) have muddy sections year-round and are often impassable in the rainy season. The mainroads between major cities should be possible by roadbike. Although it might get difficult due to landslides, road construction, missing bridges... anytime.
Always ask around before going somewhere. Make sure you ask several people (truckers, police, gas station, bus drivers) since the answers are often just guesses and people think a motorcycle can get through everywhere.
Bolivia is a very fascinating country, enjoy it.
Werner
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