A proper crimp connection cold-welds the wire to the connector. There will be an excellent mechanical and electrical connection.
Whippet, if your team is soldering wires to connectors and still having them come out, it is time to seriously evaluate your soldering procedures and training. Though soldering is completely unnecessary if a crimp is performed properly.
Here is an article that goes into great depth on the finer points of crimping, crimping tools, pull tests, etc. It is definitely worth a quick read through. Note that the article is 4 pages long!
http://www.pbase.com/mainecruising/w...ination&page=1
Some of the article is specific to boats, but much of it translates directly to robot applications. Like others have said, a ratcheting crimping tool is absolutely key to reliable crimp connections.
Edit: in some of the forum discussions linked in the above article 'pull testing' is discussed. Most notably that pulling a wire by hand results in a tug test of around 20lbs of pull force, a fraction of the force required by actual pull-test specs.