One big disadvantage is that in asexual reproduction, the offspring is not resistant to difficult surviving conditions. They are extremely vulnerable to diseases and ailments. They don't possess a commendable gene combination!
Asexual reproduction is also known as vegetation reproduction and the major disadvantage of this type of reproduction is that the off-springs are not resistant to the environmental conditions. Off-springs are vulnerable to the extreme conditions. Secondly, there is less genetic diversity in asexual reproduction therefore, evolution may not be possible in the generations. Moreover, genetic diversity in the off-springs gives the offspring less chance of survival if changes take place in environment.
A lack of variety.