I rely on my algae scraper too.
The otos do definitely eat the algae on the glass - but they only nibble at it. I think they like young algae and give up when it gets a bit tough. They're most active on the glass when I have just scraped it - presumably eating algae which I can't even see!
My snails do often leave a clear trail on the glass - but they're never going to keep it clear.