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Just that it takes extra space and that's sometimes a constraint.

In place merge sort exists. It's hard to write tho

This is beautiful. I will try to implement this once i'm at home ...

Which decreases the space complexity but increases the time complexity.

No, the time complexity is the same: O(n log n). The author of the top answer links to his book, where you can find a proof of time complexity:

...but it increases run time. It's fine not to care on hidden constants while analyzing algorithms, but not while using them in real life