The difference is, if you make a mistake when you terraform a planet, it is likely that you can't "turn back the clock" and undo your mistakes.
A single microbe introduced to a subsurface aquifer, or surface habitat, could spread throughout that habitat rapidly, quite possibly within a decade, and soon make major changes to the planet. This might for instance lead to large scale release of methane, a greenhouse gas.
The simple approach takes far longer than expected. Chris McKay estimates 100,000 years for this simple approach to reach a breathable atmosphere, not 1000 years.
All the CO2 gets turned into limestone or other forms of calcite or similar minerals, and is permanently removed from the atmosphere and is extremely hard to release again
It loses its CO2 into space. We don't yet know how early Mars lost its atmosphere, this is what Maven may clarify. However, high energy particles from the sun can strip a planet of its atmosphere, and on Earth these are kept away by its magnetic field, so it might be because of the lack of a magnetic field on present day Mars.
It loses water continually from its upper atmosphere due to dissociation into hydrogen and oxygen.
The biological cycles are unstable, or go in a direction not suitable for humans e.g. creates a stable atmosphere, but one with high levels of gases poisonous to humans such as H2S or methane, or the planet goes into a deep freeze again with biological cycles reinforcing the deep freeze (e.g. causing clouds that reflect the sunlight away).
Some purely biological issue - a micro-organism or higher organism evolves on our transformed Mars which is either hazardous to humans, a disease or an allergen, or damaging to our crops or animals, etc. Or Mars already has such micro-organisms on it.
It has much less nitrogen than Earth, which is important in our atmosphere as a buffer (77% of the atmosphere), as well as needed by many plants.
I am not saying that it is impossible to terraform Mars. It is surely a huge challenge though.
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