The risk to productive placement is that the product could be associated with something that is inappropriate or provides negative publicity.  People may see a character they dislike drinking a popular soda and therefore dislike the soda.  The benefit is that they might have the opposite reaction.  They might see a character they like associated with a product they will be more likely to use.  Of course, they say that any publicity is good even if it is negative publicity.  Product placement forces consumers to see the product in the way an ad cannot.  

And, if the movie is a hit or very popular the advertising value could be enormous.  For example, in the movie Happy Gilmore there is a placement ad for Subway sandwiches.  To this day if I see the movie on tv I get an image of Subway sandwiches.  Whatever Subway paid for their role in the movie, I'm sure it has had an exponential return.

The major benefit of movie product placement is that it gets the product in front of people without doing so in a very obvious way.  Instead of having a separate advertisement, you can have characters in a film using the product in a way that is natural.  This might be more effective in a subtle way than an actual ad would be.

Another risk is that the film might not be successful, and not that many people will see the placement. Or, even worse, the product may be associated with a major flop at the box office, or a movie that is simply not that good. In this case, the product placement would bring negative publicity. Still, advertisers may make the calculation that any visibility for their product is by definition a good thing. 

The risks to marketers in placing certain brands in movies are that the product may not be appropriate for the movie, say condoms in an animated movie (a parent may have an issue with this). The benefits are that some children may see a specific product in a movie and desire the product (think Happy Meals and action toys from the movies).