The Short Answer
The choice isn't about two ideologies, but rather two types of decisions. Some decisions incur exponentially higher change costs over time—like data models, permissions, and integrations—and need to be locked down early. Others have low change costs—like screens, fields, reports, and wording—and are best discovered iteratively.
Therefore, the model that works for most Salesforce projects is hybrid, not out of compromise, but due to its inherent structure: a fixed framework with iterative content.
The Three Determining Variables
| Variable | Pushes towards early planning | Pushes towards iterations |
|---|---|---|
| Process Clarity | Structured and documented process | Changing or undefined process |
| User Availability | Low, limited time | High, weekly review possible |
| Regulatory Exposure | Audits, compliance, approvals | Minimal |
The second variable is more crucial than commonly assumed. Agile without user availability isn't Agile; it's a series of sprints where nothing gets reviewed, and all feedback arrives at once during UAT.
The Hybrid Structure in Practice
The effective approach divides the project into two parts with different paces:
Framework Phase (4-6 weeks, planning-intensive) - Data model, permissions and visibility model, integration mapping, migration strategy, and definition of processes for the first wave. Deliverables are documented and approved.
Delivery Waves (2-week sprints) - Each wave delivers a complete scenario for a user profile, including testing and feedback. Changes within a wave do not require re-approval as long as they don't impact the framework.
The rule that holds this together: A change to the framework is a managed decision; a change to the content is ongoing work. Without this distinction, every small request goes to the steering committee, and every structural change slips under the radar.
Where Each Model Breaks
Pure Waterfall breaks at UAT: The gap between what was written in a document six months ago and what the user expects is discovered too late for a cheap fix.
Pure Agile breaks at the data model: After six sprints of local decisions, it's discovered that the structure doesn't support cross-process reporting, and fixing it requires a migration.
Hybrid breaks when the framework isn't truly finalized—when it's a "framework" in name only and is reopened in every wave. Then, you end up with the disadvantages of both models.
What to Measure Along the Way
Three metrics are enough to know if the model is working: the ratio of completed items to reopened items, time from user feedback to fix, and the number of changes that impacted the framework. An increase in the third metric is the earliest sign that the initial planning was superficial. The connection to scope management is detailed in Scope Creep and Change Control, and timelines in Salesforce Project Timeline.
Summary
The right question isn't which methodology is more modern, but rather which decisions in this particular project will be expensive to change later. Those who can answer this will almost automatically arrive at the delivery model—and it's almost always hybrid, with a clear boundary between framework and content.