Like most of the issues, the solution requires experience to know when you are at the Goldilocks point (Just Right). This specific issue has a lot in common with managing database migrations in django or any other migration system.
The ideal situation is to create migrations that can always be rolled back, but sometimes this is not possible to do operationally. For example, a schema change that restricts a field from NVARCHAR to INTEGER can only be generically rolled back if all of the unconvertable data is persisted. This can be mitigated by structuring the database to avoid these dead-ends, and that really is only gained by hard-won experience.
The problem with undoing operations via new events is the same thing -- unless you have foreknowledge of this kind of problem, it is very easy to accidentally create events that perform un-undoable actions. A very simple example of a problematic event would be something that modifies a foreign-key relationship -- let's say it's a digital asset in a game and you want the ability to transfer ownership of the item from one player to another.
The simple solution is a event like SetAssetOwner(asset_pk, player_pk). This would set the item's player foreign key field to the player's primary key. Easy. However, you have lost knowledge of where the asset came from and cannot undo this operation. A better solution would be to make an event SwapAssetOwner(asset_pk, owner_pk, recipient_pk). Yes, the owner_pk is technically redundant, but it provides a check against someone trying to steal an item with a maliciously crafted SetAssetOwner event that performs no checking. Even better, this operation can be reverted by sending the same event with the owner and recipient arguments reversed. Since these properties are part of the event message, they will be persisted in the event history and all of the information to undo the event is self-contained.