This is discussed extensively on the Ports boards. Shore Excursions Group is a broker, not a tour provider. That means they usually get you good prices, but then they take your money and contract with a low bid local provider.
Many tours will go off without a hitch, but when there is a problem, then there is frequently compounded problems communicating with any one. Viator is another frequently mentioned tour broker.
The strong recommendation from many of us who travel in Europe a fair amount is deal direct with a recommended local provider, not a tour broker.