This request is being tracked already:
Issue 52 – Use file hash as criterion for synchronization
You can vote for it there.

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Btw, I'm not sure I understand your first point (against time criteria).

Of course, time / size are easy to compare, since FTP readily reports both, but won't report a file hash value.

To implement a hash comparison, you'd calculate file hashes locally and store in a file (xml, or similar) and post on the server. If the file exists on the server, you have all the hash values for local comparison, the client only posts the files with new hash values, updates the hash data file and posts it to the server too. If the hash file doesn't exist, then use timestamps / size the first time, and calculate the hashes for subsequent users.

The synchronize remote functionality currently supports comparing files by size or date/time. These options are pretty good, but each has some minor problems

• Time only works if you are releasing from a single computer. If you're working in a team, and a file has an newer time on one computer, then the file is synchronized unnecessarily even if it has the same content.
  
• Size only works if the file actually changes. If you make a change that doesn't result in a change in file size, then the file is not synchronized.
  

I'd suggest adding a 3rd option, where file contents are hashed on both local / remote computers and only synchronized if the hash differs. This gets around both issues.