Well it can be! We've recently moved from dry-hopping all our beers in CT to dry-hopping in FV primarily with some beers getting a second dose in CT as well... double dry-hopping they call it nowadays I hear!
The issue with FV dry-hopping is that it's done with the beer is at fermentation temperature when it is still fermenting, albeit activity is tailing off and is around 20c. OK, you may think, what does that matter? Well, quite a lot....
If you look at the graph below you'll see that CO2 is much less soluble in warm liquids than cold ones, meaning, basically, that the liquid can hold less CO2 in solution at a higher temperature. As the beer is fermenting there is still a lot of CO2 being produced by the yeast and it's bursting to get out of the liquid one way or another.
This would all be fine, but we're about to add the dry hops; when hop pellets are added to the beer they break down into constituent little bits of hop which create millions of "nucleation points"; these sound great but are actually a massive pain in the arse for a brewer... picture those lager glasses in pubs which have a stream of bubbles coming up from the bottom; if you examine these you'll see there is a printed area or blob of glass which acts as a nucleation point to bring CO2 out of solution and therefore forms the stream of bubbles... now transpose that thought into millions of little nucleation points (the bits of hop) and you can imagine how much CO2 is suddenly brought out of solution, and it's got to go somewhere!
So, with all this CO2 suddenly being released out of solution and the yeast head still sticky and prominent, the beer froths up. Sometimes it overflows the FV and goes everywhere, at other times it's a bit more subdued! We tried a new technique today of adding a portion of the hops first (20%) to get things started and then planned to add the rest 10 minutes later but, after nothing happened, we threw the rest of the bag in thinking that was that.... bad mistake! Suddenly the yeast head on the FV began to churn and bubble alarmingly, then started to rise.... and rise.... and rise.... until it reached the very top of the vessel, but luckily for us any repetition of previous yeast sweeping from the floor below was prevented as the photo below is as far as it got before receding back!
Lesson learned, add a bit first then wait half an hour!