To get summary of new/missing files, and which files differ:
diff -arq folder1 folder2
a treats all files as text, r recursively searched subdirectories, q reports 'briefly', only when files differ
a
r
q
diff -r will do this, telling you both if any files have been added or deleted, and what's changed in the files that have been modified.
diff -r
I used
diff -rqyl folder1 folder2 --exclude=node_modules
in my nodejs apps.