The job of the implementation project and team (generally comprised of power users and staff from the vendor or certified partners) is to find those edge cases and automations so they can be dealt with. Most ERP has halfway decent workflow automation tools and development tools that make small modifications easy and large ones at least possible, though for huge gaps an organization might just choose a best of breed 3rd party tool and write bidirectional ETL jobs using APIs.

The real problems are

1) organization staff on the implementation who don’t actually know enough edge cases etc. This can be through poor requirements gathering ir because the power users are considered too important to take off their normal work and the company is too cheap to hire enough temp staffing to cover their absence.

2) A bad “fit-gap” analysis the maps old system functions to the new system to find the gaps. This is the vendor/partner (consultants) job so that’s on them. Failing here sets the project up for failure or major cost overruns or just years of people pissed if that they can’t do what they need to do. It also breeds shadow systems and work arounds that make knowledge transfer after staff departures vastly more difficult.

3) vendor/consultants looking to maximize $$ by either sandbagging hours of work with unnecessary things like customizations that are just not needed. I’ve seen plenty of examples where vanilla functionality is much faster than the old system but “that’s not how we do it here” . The vendor doesn’t care to push back because they’re getting paid for the extra work, and when the customer runs out if its 1,000 hour block of custom dev time and still has essential work undone the customer has no choice but to pay up or suffer. Meanwhile the vendor has every email and change request order signed off in their records where the customer approved the work so they just shrug their shoulders.

4) A general unwillingness by the customer to actually pay the $ required to do these things right. There seems to be a sense in higher level leadership that “it’s software, how hard is it to install software?” and have no clue and don’t care to hear it until the sky is falling that there’s a lot more to it.

There’s more too, but I’ve been on a few of these and never seen one that didn’t include some combination of the above. They absolutely can go smoothly with a minimal amount of the above but it takes real work and a dedicated procurement & vetting process just to find the right outside implementation partner (going with the vendor itself is sometimes okay, sometime not. I wouldn’t touch Oracle Consulting ever ever.)

Even then it will not always be as smooth as using a legacy erp with roots in the early 80’s that has been customized for decades. But that sort of tech debt is massive. New functionality can be a nightmare, causing more tech debt and raising maintenance costs non linearly. If it needs regular updates for things like federal compliance (tax systems or other regulated areas) a vendor might when it finally EOL’s a system it first released before you were born still provide those updates to remaining customers of their legacy product to give extra time to transition but then you need to either role your own dev team to do it or pay outside consultants to do so: Retirees from places like SAP often make a nice bit of extra income doing dev work like this.

TLDR: Too late, you already read this far.