Single celled organisms are easier to evolve than multicellular organisms- for obvious reasons, but this doesn’t mean they are EASY to evolve. At present we have only one known example of this happening, the earth. If the conditions for life to evolve are narrow- and we don’t know this one way or the other- it is entirely possible that life origins are excruciatingly rare events. Until someone outlines a compelling pathway for life to evolve that does not rely on chaining together a series of major improbalities, I think we should err on the side of caution and assume that originating life from an abiotic environment is not going to be an easy nut to crack. On top of this there is the “wipeout” problem. If enough large objects hit the planet (for example the kind of collision that split off the moon), or if there were planet wrecking volcanism, or no protection from cosmic radiation etc. the likelihood of life surviving goes down. There are many reasons why a palnet, even one like the earth (see Mars and Venus) could be inhospitable to life, and I don’t think w have a handle on these kinds of events at all, so again, we should assume that even when life evolves its survival may be tenuous.