Two of the main advantages of being a unicellular organism as opposed to a multicellular organism are the ability to reproduce asexually and the lack of need for a complicated organ system, in which many things can go wrong. The much smaller size of a unicellular organism can work to its advantage also.
Unicellular organisms are those that contain just one single cell. There are five types of unicellular organisms: amoeba, paramecium, protozoia, viruses and diatoms. Unicellular organisms cannot be seen to the naked eye. This gives them a great advantage when it comes to would-be predators. Most unicellular organisms reproduce asexually, which gives them the distinct advantage of not having to waste resources and energy trying to find a mate or caring for their young.They can also multiply at a much higher rate due to the fact that they are asexual. This, combined with their invisibility, makes it possible for them to thrive in almost any environment.
Unicellular organisms also have the ability to take in energy by diffusion/osmosis. This eliminates the necessity of finding food and having to have a digestive system. Also, almost anything can be a potential food source for a unicellular organism. Beyond the digestive and mating systems, unicellular organisms also have no need for circulatory or excretory systems. In other words, there are very few things that can go wrong with them.
Unicellular organism may miss out on many of life's higher pleasures, but from an evolutionary standpoint, they are well suited to thrive.