The bus may be a noisy and uncomfortable ride where you have to share with other strangers; thereby having a lack of privacy.
Buses are also famed for being late and not stopping where you want them to stop. They can also be expensive depending on where you want to go. As well as this the trip may take a longer route than you'd go by private transport.
Buses are also sometimes full meaning you can be left waiting for the next one. They are also less environmentally friendly than a bicycle.
So as is clear to see there are a number of advantages and disadvantages to buses depending on the individual service, where you are and how you want to use them.