"the output if you dump that into the search field, versus if you click on a label name, don't match if you have more than about 30 messages in that label. Go try it."
The output is different because it only shows me 20 rows per page when I search on label:label, versus 100 rows if I click the label, but the results are the same. Were you seeing something more broken that the number of results per page?
FWIW, I've tried it on a few labels, none having fewer than 10,000 messages in them.
There are many subtle differences: as you noted, the rows per page differs; clicking to select all offers "Select all conversations that match this search," rather than "Select all conversations in <Label>"; the "Remove label <Foo>" button disappears; an Archive button appears; "Move to" disappears, but "Move to Inbox" appears instead; etc. It's not broken in the sense that it doesn't work, but it's broken in the sense that Google's implying an equivalence that is not there.
The output is different because it only shows me 20 rows per page when I search on label:label, versus 100 rows if I click the label, but the results are the same. Were you seeing something more broken that the number of results per page?
FWIW, I've tried it on a few labels, none having fewer than 10,000 messages in them.