Access to a large electric system provides big economies of scale not only in terms of diversity and low cost of power generation, but even more so for minimizing necessary reserves and for making it easier to balance variable resources like wind and solar.
Islands without access to neighboring electric systems are in every aspect on their own. They have to generate all the power they need and they must themselves keep the system reliable no matter what. Adding more renewable energy to an island system can be an opportunity, but large amounts of variable renewable energy increases the challenge of maintaining reliability at reasonable costs.
In 2014 there were 3634 outages in the US electric system according to the Eaton Blackout Tracker. It affected in total over 14 million people. On average close to 4000 people were affected per outage, which on average lasted 43 minutes. 30 % of the outages were caused by weather and trees. 28 % were caused by faulty equipment and/or human error.
Almost all outages were at the distribution system level, Outages at the transmission level are very rare, but when they happen the consequences are bigger, affect more people and take longer time to restore. The Northeast Blackout in August 2003 hit 55 million people in United States and Canada. One month later the Italy Blackout had also about 55 million people in Italy, Switzerland, Austria, Slovenia and Croatia losing power. As recent as in March this year 90 % of Turkey with 70 million people lost their power. The largest blackout so far was in July 2012 affecting half of India and 620 million people. In fact the grid collapsed for a second time in two days.