I suspect the downside to having employment-at-will, very modest unemployment benefits, and a liquid labor market exists. I also suspect that it can be measured in terms of objectively verifiable criteria, and compared with the alternatives. For example, I suspect that I can look in a US newspaper for a count of the number of cars burned last night by immigrants rioting over lack of economic opportunity. It will take me a whole lot of searching, because few newspapers publish articles titled "BREAKING! No riot today, either!"
Then I could look at the same number in a French newspaper. (100 a day on quiet days, spiking to over a thousand on New Years or during periods of mass unrest. You might think I'm exaggerating. http://www.time.com/time/world/article/0,8599,1869392,00.htm... )
Alternatively, if counting riots seems a little messy, I could look at unemployment statistics for those under twenty five. In the United States, for youths at large, it is about 11~12% right now. In France, it is over 20 -- down a bit from a few years ago when it hit 25%. For socially disadvantaged groups in America, for example black males without a high school degree, it is about 48%. For similarly situated folks in France (for example, uneducated immigrants), unemployment is close to total.]
I can go on if you want me to.