I have been running Owncloud since 2011 without any reverse proxy and it's been fine.

I'm still on an old version because every time I have tried to upgrade it, it has fucked up, and i've wasted loads of time getting it back running again. New versions /should/ be better I hope!

Pick decent user passwords, protect it with SSL, and have a local firewall on your server that only allows required ports though (80 and 443), and you should be fine.

I take a backup of mine now and again, but it's quite small - only about 50gb.