While spores seem hardened against the extremes of space, we haven’t shown that
any of this hardy life is capable of colonizing a barren world. It seems like all life on Earth depends on some already functioning biosphere. In other words, even if we sent tardigrades to a world with oxygen and liquid water, what would they eat? Where would they get nutrients such as vitamin B? All the vitamin B we consume is created by bacteria, no animal produces it on its own. So we would have to send thousands of interdependent species. And I’m willing to bet the majority of them aren’t nearly as hardy.
Sending spores to a planet that already has life might work. But I can’t help but think whatever life we introduce would be at a disadvantage. Maybe life on that planet never incorporated certain proteins, vitamins, or amino acids and whatever we send just ends up getting scurvy and dies out.