diff --git a/packages/schematics/angular/migrations/update-12/update-angular-config.ts b/packages/schematics/angular/migrations/update-12/update-angular-config.ts
index ec32330418f3..d9e91ea64ea2 100644
--- a/packages/schematics/angular/migrations/update-12/update-angular-config.ts
+++ b/packages/schematics/angular/migrations/update-12/update-angular-config.ts
@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import { JsonValue, workspaces } from '@angular-devkit/core';
+import { JsonValue, tags, workspaces } from '@angular-devkit/core';
 import { Rule } from '@angular-devkit/schematics';
 import { allTargetOptions, allWorkspaceTargets, updateWorkspace } from '../../utility/workspace';
 
@@ -30,29 +30,39 @@ const ServerBuilderOptions: BuilderOptionsType = [
 ];
 
 export default function (): Rule {
-  return updateWorkspace((workspace) => {
-    for (const [, target] of allWorkspaceTargets(workspace)) {
-      if (!target?.builder.startsWith('@angular-devkit/build-angular')) {
-        continue;
-      }
+  return (_tree, context) =>
+    updateWorkspace((workspace) => {
+      for (const [targetName, target, projectName] of allWorkspaceTargets(workspace)) {
+        if (
+          !target.builder.startsWith('@angular-devkit/') &&
+          !target.builder.startsWith('@nguniversal/')
+        ) {
+          context.logger.warn(tags.stripIndent`
+            "${targetName}" target in "${projectName}" project is using a third-party builder.
+            You may need to adjust the options to retain the existing behavior.
+            For more information, see the breaking changes section within the release notes: https://github.com/angular/angular-cli/releases/tag/v12.0.0
+          `);
 
-      // Only interested in Angular Devkit browser and server builders
-      switch (target.builder) {
-        case '@angular-devkit/build-angular:server':
-          updateOptions(target, ServerBuilderOptions);
-          break;
-        case '@angular-devkit/build-angular:browser':
-          updateOptions(target, BrowserBuilderOptions);
-          break;
-      }
+          continue;
+        }
 
-      for (const [, options] of allTargetOptions(target)) {
-        delete options.experimentalRollupPass;
-        delete options.lazyModules;
-        delete options.forkTypeChecker;
+        // Only interested in Angular Devkit browser and server builders
+        switch (target.builder) {
+          case '@angular-devkit/build-angular:server':
+            updateOptions(target, ServerBuilderOptions);
+            break;
+          case '@angular-devkit/build-angular:browser':
+            updateOptions(target, BrowserBuilderOptions);
+            break;
+        }
+
+        for (const [, options] of allTargetOptions(target)) {
+          delete options.experimentalRollupPass;
+          delete options.lazyModules;
+          delete options.forkTypeChecker;
+        }
       }
-    }
-  });
+    });
 }
 
 function updateOptions(