I believe all trading platforms has its pros and cons, leaving its choice of use to traders. With my experience using the web trader, i have noticed this few disadvantages:
Heavy load. It needs good and steady internet connection to start, and it is not advisable to open other heavy pages in the same browser simultaneously.
Over-crowded interface. At first glance, it seems bright and unobstructed, but when I tried to use it, it felt somehow cramped.