Plenty of parts of Google have done UA-sniffing all over the place. Opera, even after being Chromium based, have had plenty of problems as a result: the UA doesn't match so therefore they get an "unsupported" or "degraded" experience.
Even basic Google search seems to do some UA sniffing. I run Firefox but usually browse with IE or Chrome user agents, and clicking the results on a search won't actually navigate to them