> I don't understand how you could possibly claim the Google Drive is in any way more secure than OwnCloud.
It depends on your threat model. Against the US? It's wise to assume it's game over, no matter what you do, here.
Foreign state actors? ownCloud is seriously pwnable. not quite so with Google services (unless that foreign gov't has an intelligence sharing agreement with Google)
Criminals? Again, ownCloud is seriously pwnable.
I'm a US citizen, so I'm primarily concerned with foreign and criminal actors (knowing that it's game over versus US intelligence), therefore ownCloud does not fit my threat model.
Have to agree with OP (who did audit the code as you suggest). PHP OwnCloud feels a bit like a janky duck taped mess. I'm always a bit surprised it works and it doesn't make me feel a whole lot more secure than using a provider like MS, Dropbox or Google. I just like running my own stuff which is why I use it.
The web interface of Seafile (https://github.com/haiwen/seafile) is written in Python/Django. The server is C, though.
As far as I know, most people who quit Google do it because they dislike Google mining their data, not because they don't think Google is secure.
Threat models and risks vary. SAAS offers aggregated data and appealing targets, though they may be well hardened. Much as I criticise Google, I find their claims of protecting data reasonably credible (not enough to be comfortable with it).
A distributed system with many known and unknown vulnerabilities and a readily determined network signature (nmap or similar) remains a bulk source. A determined adversary could scan all possible network space quite quickly and access data. With known targets -- monitoring your network traffic, knowing URLs or MXs -- they could target you directly.
NB: I haven't audited OwnClowd, nor am I particularly qualified to do so.
Microsoft provides examples of both. Samba's Andrew Tridgell has commented that the team knows many ways to crash or compromise Windows systems. Microsoft's academic code licensing provided pretty liberal access to OS source code. Similarly corporate partner licensing for contractors.
Though security didn't exactly thrive under that regime... PGP is another company that provided core crypto code for public review, though it wasn't Free Software.