The following example shows a list of Computers that were missing one or more critical updates a few days ago and are still missing updates.
let ComputersMissingUpdates3DaysAgo = Update
| where TimeGenerated between (ago(3d)..ago(2d))
| where Classification == "Critical Updates" and UpdateState != "Not needed" and UpdateState != "NotNeeded"
| summarize makeset(Computer);
Update
| where TimeGenerated > ago(1d)
| where Classification == "Critical Updates" and UpdateState != "Not needed" and UpdateState != "NotNeeded"
| where Computer in (ComputersMissingUpdates3DaysAgo)
| summarize UniqueUpdatesCount = dcount(Product) by Computer, OSType