The disadvantage of the Brownfield approach
If you have multiple productive SAP instances and intend to consolidate them into a single instance, there will be challenges in creating a single consistent solution from several conflicting sources.
Over time you will also need to go to a full SAP Fiori based SAP S/4HANA approach so you will end up migrating to new SAP S/4HANA functionality anyway and taking longer to do it. This results in an increased spend over a longer period of time and delaying the benefits realized from the new solution. End users are also refrained from using the tile based SAP Fiori design that is available on any device.
SAP is no longer adding functionality to the GUI transactions and in many cases is retiring them or eroding their functionality. Therefore continuing down a dead-end path with the existing GUI is limiting opportunities to benefit from new functionality and course correct to a more standard overall solution.
Much of the new SAP Fiori designed functionality has no GUI equivalent such as Bank and BOM Management. This new functionality is typically not backward compatible with the GUI; BOM versions and usages are a good example of this.
Continuing with the GUI as an initial transition onto SAP S/4HANA may be a low risk approach however this needs to be balanced against the benefits of the SAP Fiori transactions the additional information they provide. SAP Fiori brings the ability to navigate from information provided to highlight the need for corrective action and the ability to carry out the corrective action from where you are in SAP Fiori with a couple of clicks rather than endless navigation elsewhere.
Enthusiastically adopting SAP Fiori also guides you down the path of SAP’s product roadmap while avoiding the old mistakes of going completely in your own direction.