The tags replaced labels in Yosemite.
Labels could be added or removed whatever the number of files selected, and could not be combined. Tags are another story, since you can combine them.
But if you select: FILE-1 and FILE-2 etc all the way to FILE-20, then add a tag to them, and if you do not deselect the 20 files, and go back to the File menu, in order to remove the green tag, you cannot "remove tag" on these 20 files the way it worked before (say, with labels, before Yosemite, when you would remove label on 1-∞ files at once). It doesn't work over 10 files the way it would work if only 1-10 files were selected. Remove tag doesn't work for over 10 files.
Visual explanation of the problem:
One file is selected then "add green tag" appears as an option in the Finder:
The green tag is added:
While this one file is selected, "add green tag" becomes "remove green tag" in the File menu of the Finder (or "remove tag "green"", which isn't proper English, but whatever):
Now I select 30 files without any tags:
and "add green tag" to them (notice how now all the tag circles are strangely circled with a white stroke, by the way, not sure what that's supposed to mean to the user):
Green tags gets added to 30 files without a problem:
Now I am looking for the "remove green tag" option for these files, but the Finder asks me if I want to "add tag green" to these files:
If I "add tag green" of course nothing happens, since you can only attribute one green tag once to each file, and this has already been done once. Neither does this remove the tag.
It seems that it would make sense if the command displayed said "remove all green tags in selection", but that's not the case.
I can now only de-tag my files by groups of 10 files.If I select 10 of them, then "remove green tag" in the File menu works...
Therefore, if you inadvertently tag 1503 files in one clumsy gesture, and do not have access to the Edit -> Undo "add tag green" command, how would you select all 1503 files to remove their tags in one single move instead of 150?