well, good riddance to flash, but Encrypted Media Extensions are definitly not an advantage, see for example https://www.eff.org/deeplinks/2016/04/save-firefox
EME is a huge advantage from a security perspective because it avoids pulling in a massive platform (Flash, Silverlight) just to play video. We can debate other aspects but from a security perspective moving to a small, tightly-scoped module is a big win and that's important when the cost of vulnerabilities is in the billions per year. It's also somewhat good for the web since both of those platforms are competitors…
AFAIK, You need EME to make sure that only browsers that have the Foo-Corporation EME plugin can play Foo-Corporation videos.
And to get the EME plugin for a browser, you need to ask permissions from Foo-Corporation to have it. So anyone writing a new browers (like Mozilla did with what became Firefox) will have to ask for this permission and will likely not just get it.
Seems like another example of where big companies got to where they are because the space they moved into was wide and open and free. But now that they have claimed that space and divided it up amongst a few players, they'll do their damned best to fence it of, wall it in and make sure none else can ever move into ever again.