One of the disadvantages is that only half the population
(females) is capable of. Another one of the disadvantages is that
two parents have to be involved in the process. Also having to find
a mate and producing gamete cells is also another disadvantage.
Fertilization has to take place and there is no guarantee that the
nucleus of the male sex cell would fuse with the cell. Also this
takes a long time and there is no grantee that an offspring would
Sure, all of that is true but I think a lot of people forget that plants undergo sexual reproduction as well, and they can guarantee many "seeds." (by seeds, I'm not talking about the female and male gametes but the seed [or child if you want to call it that] itself that is created. Now what happens to that seed, whether it does grow or not, it is not a fault of the reproduction system of the flower.) So really, the only disadvantages I could think of from sexual reproduction are how frequent it occurs as well as mutation and the amount of time needed.