Most mirrorless cameras do not do FAST erratic moving subjects well. The mirrorless AF technology is still developing.
This is a completely false statement.
The AF tech is the same as from the same company. Just because you remove a mirror does not mean you begin at square one.
There are a couple mirrorless cameras that will keep up with and/or beat the Canon 1DxII or Nikon D5.
Edit:
The above statement was a little short so here's a bit more.
The appearance of mirrorless not doing as well with AF is because of their origin. They were not origionally meant for fast action and therefore did not have great AF. Add to the fact most saw all of the Sony hype and equated them to be the leader in the market. Sony did have horrible AF in the beginning.
Now why I say it doesn't matter. If Nikon makes the D5 mirrorless, the ONLY difference would be an EVF (and processor for it) and battery life. NOTHING else needs to change.
So sating that mirrorless have worse "X" based soley because they are mirrorless is way to simplistic an answer and is inherently incorrect.