Typically what happens is that business discovers they can now implement every last feature they desire without getting any push back from engineering so they go wild implementing new features without realizing the consequences. There is no VCS, no QA, no testing. There is no one telling them they cannot do something because it won't scale, it isn't secure or it won't be maintainable.
Their only metric for success is that they get the result they want now and the long term consequences be damned. Worse yet, every single person using the rules engine is acting independently and not as a team. There is no code review, when the rule works to their satisfaction it gets pushed into production.
At first everything works fine and people get promoted for saving money on engineering costs but then the rules start getting more complex, start becoming composed of other rules, need to have more complex actions or need to integrate with third party systems. Eventually the simple rules engine turns into a bastardized programming language that everyone adds onto and never modifies because no one understands how a modification will affect the 4000 other rules in the engine. At that point you end up having to do a complete re-write, which is something I have had the displeasure of doing in the past.