The first issue with this legislation is its questionable constitutionality. According to the Constitution, the Electoral College is the only way to elect the President. Any changes to that process must be amended as set out in Article 5. However, the anti-College movement pushed by the National Popular Vote Campaign and states like Massachusetts seek to effectively amend the Constitution with a simple majority vote among the states. This, of course, is not one of the methods of amendments laid out in the Constitution and is simply illegal.
Many of the proponents of this legislation argue that the current system encourages candidates to only focus on "battleground" states ignore "wide swaths of the country." However, switching over to a national popular vote system will only encourage candidates to focus on densely populated, largely urban areas of the country. This would arguably disenfranchise an even larger swath of the country. You can be sure that a place like New Jersey is going to get a lot more attention from a candidate than a place like North Carolina, under a populist system. Under the current system, both have the approximately the same populations and the same number of electoral votes. So, they both have equal influence in the election. Under the populist system, North Carolina would be ignored and New Jersey would get all the attention.
There's also the issue of the disenfranchisement of the people of Massachusetts. Under the current system, their votes ultimately determine their state electoral votes. Under this new system, the people of Massachusetts don't actually have a say in the way that their state votes. The state electoral votes are determined by the national at large, not the people of Massachusetts. But of course, it's impossible to have this sort of debate on the electoral process when one side is attempting to short-circuit the national conversation by subverting the law.