The only problems (all of which can be avoided with proper planning) are:
1. Suck back. This is an issue with any cold crash, not just long ones. Avoid it by switching to an S-shaped one-piece airlock. Also, using vodka instead of water or star-san works, but vodka evaporates faster than the other two, possibly leaving you with:
2. Airlock drying out. This is an issue with any extended aging process, and is exacerbated during long cold crashes because the relative humidity of the fridge/freezer is so low that evaporation happens faster. S-shaped airlocks dry out more slowly that 3-piece, so that helps. Also, vodka evaporates faster than water or Star-san solution, so again, this can be avoided with a classic S-shaped airlock and water or Star-san.
3. Slow bottle conditioning. Cold crashing is not an issue if you keg and force-carbonate, but if you are bottling, one might ask why you are cold crashing. By cold crashing (and even worse if you are cold crashing and adding finings like gelatin), you are taking the bulk of the suspended yeast out of solution. So if you want to bottle condition, the small amount of yeast left in the beer will take longer to revive from dormancy, begin eating, reproducing, and converting the sugar to CO2. Which make s me ask "Why are you cold crashing?" It is a completely unnecessarily step except for aesthetic purposes, and to a lesser degree, long-term stability (my beers rarely last more than 2 months, so I never worry about long term stability).
IMHO, if you are bottle conditioning, you are only complicating your life and brewing process by cold crashing and/or adding finings. And again, the only real reason to do either is aesthetic - you want a clearer beer. But ask yourself who cares if you have a little haze in your beer? it doesn't affect flavor.
So, long story short: an extended cold crash won't hurt a thing as long as you A: don't suck back anything you don't want in your beer; B: don't let your airlock dry out, or C: don't plan on bottle conditioning.