Designing a customer facing offer claim web-form, and have arrived at two major directions for the design.
In one the user progresses left to right with the form summary / progress bar displayed across the top.
In the other version the user is going from the top down, with the questions acting as both summary and progress indicators.
Which do you think is the clearer navigation model ?
Jackob's law would suggest the more traditional layout is easier. But the feedback from stakeholders and users has been strongly in favour of the vertical layout.
What do you think might be the downsides of the vertical layout ?
At first: Use the feedback of the users! (not necessarily the stakeholders if they do not use the form by themselves).
I think both navigation Models are equal. The most important thing is, that the steps are clear to the users. In your example there are multiple steps per major step. This might be confusing. Also the major steps have quite long forms which makes the user scroll down. Therefore the user navigates in two directions. In the vertical version the user does not have to bother with the major an minor steps it's just steps.
If there would be only a few steps with a snackable amount of fields in each step the horizontal model has the advantage that the user is always aware of the progress. In the vertical version the user would have to scroll down to see the progress because the open form pushes the upcoming steps out of the viewport.
These ar just some thoughts on the issue. But as I said in the beginning: Listen to the users. You don't have to find arguments to disprove the users.