The most important questions fall into five areas: does the platform adapt to your process or force you into its own, do finance and operations actually plan on one model, how real is the scenario simulation, will planners actually use it instead of falling back to spreadsheets, and can it connect to and govern your data at scale.
IBP is a process first, owned by the business, not a piece of software. What you're evaluating is the IBP software, or solution, that runs it, and evaluating it starts before you contact a single vendor.
IBP maturity measures how far a company has moved from siloed, reactive planning toward a single, financialized operating model. The stages, and where most companies stall.