To model a planet's environment is an incredibly complex process, one which could take millennia. This alone would make terraforming even a small planetoid a daunting task. But terraforming is not just about making the physical conditions of the planet's environment similar to Earth's. It also involves the ecosystem.
The ecosystem of Earth as we understand it today consists of billions of species, all interacting with each other in ways that we sometimes do not understand. As soon as one species is removed, all the organisms interacting with it are affected, and thus like a giant circle of dominoes all of the species in the entire ecosystem are affected, usually adversely. Usually old species or new ones take the place of the eliminated species, and the ecosystem survives.
The majority of the species that make up our ecosystem, including ourselves, cannot survive without the support structure that all of the species provide for them. In an attempt to colonize another world, we would most likely want to make it self sufficient, which would in part involve growing our own food there. To do this we would have to create a new ecosystem based on parts of the whole ecosystem of Earth, as alone the organisms we depend upon for survival cannot survive. But which organisms to take?
Certainly, no one wants the mosquitoes. And who wants to be stung by bees? Weeds are a nuisance, and wolves threaten livestock. So we don't take them. Spiders are adversely affected due to lack of mosquitoes to get caught in their webs, flowers die out from lack of pollination, grasses become the same nuisance that weeds once were on old Earth as they expand to fill the evolutionary niches left out, and small animals become rampant as there are no predators keeping their population down. Obviously, that's not the solution.
There is no way to know just what might happen if an ecosystem is not complete. Earth's ecosystem evolved on its own, not out of fragments of another. There is one sure way to produce some sort of lasting ecosystem, but it involves millions of years, and starting with simple unicellular organisms. We cannot wait that long to colonize new worlds. The only other option left to us is to simply put up our best effort in trying to build Earth's exact ecosystem the best we can, and hope that the holes and problems that we leave will fix themselves in a way that will not hinder us.