Fyi, I've been using gnomio since 2013. Very satisfied with and thankful for the service.
I've used it for 2-4 university classes per semester, with up to 90 students per class. Never had a problem with student numbers or the size of my files.
In the first year, the only problem was a great deal of inconsistency with the server sending notification emails. No idea why. It could have been me. But it hasn't been a problem since then.
If it were a bit more affordable, I would have been happy to pay to remove the ads. But I never did and the students never complained (although I would not have been disappointed if they had).