The disadvantages are that you only own a 25% share of the house, therefore if you sell you only get 25% of any profit. Many are structured so you take the loss rather than the scheme. Lets say you purchase for �100K, your share being �25K, if the house drops in value to �75K it's effectively cleared your money out. If you can to sell in many cases you would be liable for that short fall.
Also in that instance you would also find it difficult to sell becuase you don't acutall own all of the house.