Updating a formula field with a Custom Action

I have a Custom Field, which is a formula field.  I can't remove the formula aspect of the field to make a switch to updating via workflow rule and the field is too integral to our processes and project structure to easily replace it with a non-formula equivalent (it's displayed or filtered in 90-some% of our reports and workflow rules).

So instead, to try and avoid all that surgery, I want to start updating the formula field with a Custom Action (we have one already that updates the Task that the formula field is tied to) because the formula seems to be unreliable (sometimes it updates immediately, other times it never gets around to updating). 

Is there any reason *not* to use a business rule (CA or WFR) to update a formula field?  I understand that the formula, when it does get around to calculating, would override the dropped-in value but it should just be the same value that was dropped in anyway.

I'm just looking for any "I did that and my whole environment caught fire" cautionary tales before I pull the trigger on this.