Conducting stakeholder analysis and RACI documentation is indeed helpful in getting everyone onboard with what the project roles and expectations are. And its not just helpful with the core project team but even the support team ( or external entities ) which might be consulted on the project and be required as and when needed.
Its not only the executing team that faces this issue, its even in the project / product being worked on. Business Process Modeling is not given as much importance as it should. After all the product / project success depends on the seamless flow of data from one entity ( role ) to another and identifying the roles and actions is crucial to understanding which teams are impacted or need to be communicated. Swim lanes seem pretty simple but as far more effective if done right. They ensure we haven't missed any stakeholders and the data handshakes are understood by all.
The fall out of not including the right roles and their tasks at the outset, could vary from very serious to moderate consequences but in all cases it pushes the project into a fire fighting mode during / after completion with people running against the clock and budget to fix the misses. The last minute surprises not only dislodge the project, it also could lead to reduced output quality as hasty fixes / band aid solutions are strapped on without rigorous testing.
A miss could also be as simple as not keeping the impacted role in the loop which means they don't understand what needs to be done , when , where and why. Communications plays a vital role - a successful project always has open channels of communication, a visible dashboard to track progress and clear release plans , release notes as well as defined post implementation support.
Post implementation plan is most often the most neglected part of any project undertaking. Everyone is so focused on getting the product / project out of the door that they forget to consider what needs to happen when its actually out of the door. Admin / support staff is identified at the last minute and given little to no training which means they scramble around to provide customer support. Which is why the project was undertaken in the first place - to enhance the customer experience or add value to existing processes / features.
So in the end the project is as successful as the team behind its steering wheel and if the right people are involved in it from the start its highly unlikely to go offtrack.