The biggest problem about panspermia I think is this:
We will examine in the next week or two
some of the steps that must have occurred
if life arose "from scratch" on Earth.
There are a lot of complicated and
possibly unlikely steps that must have
occurred.
Many people argue that because the odds are so
small for all of these steps to have occurred
in the right time in the right place, panspermia
is required to explain life on Earth.
This seems to me to be a big cop-out, because
you still have to have life arising somewhere,
sometime!
There is a possible compromise, which is
that some of the stages of the origin of
life occurred elsewhere in the galaxy.
Some advanced organic molecules (for example) --
complicated, and necessary for life, but
not yet living -- could have been transported
to Earth. Here the final stages on the
road to "alive" took place.