The biggest caveat of MU-MIMO is that it works only on the downlink connection: traffic being sent from the AP to your laptop, tablet, smartphone, and other Wi-Fi devices. MU-MIMO doesn’t directly improve the wireless speeds of uplink connections: traffic flowing to the AP (such as an upload to the Internet).
It’s also worth repeating here that the only way to gain the full benefit of MU-MIMO is when the technology is supported on both the router and the device that’s connecting to the router. So in addition to having an 802.11ac adapter onboard, the client must explicitly support MU-MIMO—and there are precious few adapters meeting that criteria today.
Finally, MU-MIMO works best with stationary Wi-Fi devices. If you’re walking around your home while watching a video on your smartphone or tablet, you’re not going to get the full benefit of MU-MIMO even if that device supports it. Your router might even limit that connection to using SU-MIMO, so that the connection doesn’t negatively impact stronger MU-MIMO connections.