Beyond that, though, the penalty for a mishap is easily remedied, you can get the untouched soil from your demiplane (I'm assuming Genesis is a non-core spell), and the temporal drift is negligible. But yeah, if your DM rules that killing the wizard's mother doesn't destroy him in the present or that killing him just leads to him being replaced by another wizard or something, then it's not going to be an effective means of assassination.