[D] When to transition from simple heuristics to ML models (e.g., DensityFunction)?

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Two questions: What are the recommendations around when to transition from a simple heuristic baseline to machine learning ML models for data? For example, say I have a search that returns output for how many authentications are “just right” so I can flag activity that spikes above/below normal. When would I consider transitioning that from a baseline search to a search that applies an ML model like DensityFunction? Any recommendations around books that address/tackle this subject? Thx submit...