What a Geocortex site hides from the map
Layer count is not complexity. The expensive parts of an Essentials migration live in places an inventory taken from the map will never see.
Every migration inventory starts the same way: open the map, count the layers, count the tools. It is a reasonable first pass and it will undercount the estate every time. These are the things we now look for on day one, because each one changed an estimate after the fact.
Dead configuration
A Print button declared in a toolbar that never renders. A module with isEnabled: false that still owns three commands. A site flag saying there are no print templates while the viewer shows a print tool. Every one of these gets a row in the register with a zero estimate and the flag cited as evidence. Skipping them is how a feature list grows by a third with nothing behind it.
Things that break the export tool
VertiGIS ships an Export Web Map tool that publishes an Essentials site as an ArcGIS web map and saves days on a large layer estate. It fails outright on command hyperlinks in feature descriptions, it drops data links entirely, and token-secured standalone ArcGIS Server services prompt every user for credentials afterward. Find those three before promising the time saving.
Assets that 404 after decommission
Layer icons and images referenced through the Essentials virtual directory live on the Essentials server. The day it goes, every one of them is a broken image. A small rehosting task, but one that has to be in the runbook rather than discovered in UAT.
The rule we use
Priority is uncertainty times blast radius, not difficulty. Anything that repeats across the whole estate, or depends on something the client controls, gets assessed before anything easy. That order is what turns an inventory into a quote you can defend.