Disadvantages from the buyer's point of view or the seller's?
The buyers will do all the hard work, like building a brand and SEO, and in the end it would all go to waste when the contract is over.
The seller's disadvantage would be if they get a really high offer for the domain they will not be able to accept it because they've rented out the domain. And if the site that is being developed on the domain is of low value, it may hurt SEO rankings in the long run.