The strongest argument for situation ethics not being a practical method of moral decision making is that it relies on predicting the consequences of an action (it is consequentialist). It can be hard to predict the outcome and take every possible situation into account, this will hinder things if a decision is needed quickly, or it can make it impossible to come up with a decision if the long term consequences are unable to be predicted.
Another reason that Situation Ethics is not a practical method of decision making is that Situation Ethics sometimes requires a type of decision making that some people do not have, this applies if people are unaware of the full situation, or if they are unable to comprehend the full situation that they are in, or have been told to make a decision on.
However, Situation Ethics emphasises pragmatism as one of the four working principles, this mean that the action that you choose has to work - have a loving outcome - in order for it to be justified correctly. This means that people have to consider the practical outcome of the action for everybody involved and see what is the best way to achieve the most loving result, therefore it allows us to consider a greater good.
On the other hand, love is a subjective thing, it has such a wide meaning that some people even exactly the