That definitely seems to be getting less reliable these days. A number of times I've found it refusing to work, or redirecting me to the primary UI arbitrarily, a few months ago there was a time when you couldn't login via that UI (though logging in on main and going back worked for me).

These instances seem to be temporary bugs, but they show that it isn't getting any love (why would it? they only maintain it at all under sufferance) so at some point it'll no doubt be cut off as a cost cutting exercise during a time when ad revenue is low.